On the Origin of Species, 6th Edition
by
Charles Darwin
Part 8 out of 11
doubtful forms or sub-species and some as distinct yet closely allied
species representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement of
our imaginary Glacial period, the arctic productions were as uniform round
the polar regions as they are at the present day. But it is also necessary
to assume that many sub-arctic and some few temperate forms were the same
round the world, for some of the species which now exist on the lower
mountain slopes and on the plains of North America and Europe are the same;
and it may be asked how I account for this degree of uniformity of the
sub-arctic and temperate forms round the world, at the commencement of the
real Glacial period. At the present day, the sub-arctic and northern
temperate productions of the Old and New Worlds are separated from each
other by the whole Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been still
more completely separated from each other by wider spaces of ocean; so that
it may well be asked how the same species could then or previously have
entered the two continents. The explanation, I believe, lies in the nature
of the climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that the
climate was warmer than at the present day. Hence, we may suppose that the
organisms which now live under latitude 60 degrees, lived during the
Pliocene period further north, under the Polar Circle, in latitude 66-67
degrees; and that the present arctic productions then lived on the broken
land still nearer to the pole. Now, if we look at a terrestrial globe, we
see under the Polar Circle that there is almost continuous land from
western Europe through Siberia, to eastern America. And this continuity of
the circumpolar land, with the consequent freedom under a more favourable
climate for intermigration, will account for the supposed uniformity of the
sub-arctic and temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view, and
to infer that during some earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate southwards
as the climate became less warm, long before the commencement of the
Glacial period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United States.
On this view we can understand the relationship with very little identity,
between the productions of North America and Europe--a relationship which
is highly remarkable, considering the distance of the two areas, and their
separation by the whole Atlantic Ocean. We can further understand the
singular fact remarked on by several observers that the productions of
Europe and America during the later tertiary stages were more closely
related to each other than they are at the present time; for during these
warmer periods the northern parts of the Old and New Worlds will have been
almost continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would have had to compete with
them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification--for
far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on the
arctic lands of Europe and North America. Hence, it has come, that when we
compare the now living productions of the temperate regions of the New and
Old Worlds, we find very few identical species (though Asa Gray has lately
shown that more plants are identical than was formerly supposed), but we
find in every great class many forms, which some naturalists rank as
geographical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which, during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in marine areas completely sundered. Thus, I think, we can
understand the presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied
crustaceans (as described in Dana's admirable work), some fish and other
marine animals, inhabiting the Mediterranean and the seas of Japan--these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America, the
Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain
that such species have been created alike, in correspondence with the
nearly similar physical conditions of the areas; for if we compare, for
instance, certain parts of South America with parts of South Africa or
Australia, we see countries closely similar in all their physical
conditions, with their inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the plainest
evidence of the Glacial period, from the western shores of Britain to the
Ural range, and southward to the Pyrenees. We may infer from the frozen
mammals and nature of the mountain vegetation, that Siberia was similarly
affected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly
covered the central axis, and fed glaciers which rolled 4,000 feet down the
valleys. The same observer has recently found great moraines at a low
level on the Atlas range in North Africa. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on ancient and gigantic
moraines. Southward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr. J. Haast and Dr.
Hector, that in New Zealand immense glaciers formerly descended to a low
level; and the same plants, found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold period. From
facts communicated to me by the Rev. W.B. Clarke, it appears also that
there are traces of former glacial action on the mountains of the south-
eastern corner of Australia.
Looking to America: in the northern half, ice-borne fragments of rock have
been observed on the eastern side of the continent, as far south as
latitude 36 and 37 degrees, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46 degrees. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a vast
mound of detritus with great boulders, crossing the Portillo valley, which,
there can hardly be a doubt, once formed a huge moraine; and Mr. D. Forbes
informs me that he found in various parts of the Cordillera, from latitude
13 to 30 degrees south, at about the height of 12,000 feet, deeply-furrowed
rocks, resembling those with which he was familiar in Norway, and likewise
great masses of detritus, including grooved pebbles. Along this whole
space of the Cordillera true glaciers do not now exist even at much more
considerable heights. Further south, on both sides of the continent, from
latitude 41 degrees to the southernmost extremity, we have the clearest
evidence of former glacial action, in numerous immense boulders transported
far from their parent source.
>From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres--from the period having
been in a geological sense recent in both hemispheres--from its having
lasted in both during a great length of time, as may be inferred from the
amount of work effected--and lastly, from glaciers having recently
descended to a low level along the whole line of the Cordillera, it at one
time appeared to me that we could not avoid the conclusion that the
temperature of the whole world had been simultaneously lowered during the
Glacial period. But now, Mr. Croll, in a series of admirable memoirs, has
attempted to show that a glacial condition of climate is the result of
various physical causes, brought into operation by an increase in the
eccentricity of the earth's orbit. All these causes tend towards the same
end; but the most powerful appears to be the indirect influence of the
eccentricity of the orbit upon oceanic currents. According to Mr. Croll,
cold periods regularly recur every ten or fifteen thousand years; and these
at long intervals are extremely severe, owing to certain contingencies, of
which the most important, as Sir C. Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last great
glacial period occurred about 240,000 years ago, and endured, with slight
alterations of climate, for about 160,000 years. With respect to more
ancient glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations, not
to mention still more ancient formations. But the most important result
for us, arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly through
changes in the direction of the ocean currents. So conversely it will be
with the northern hemisphere, while the southern passes through a glacial
period. This conclusion throws so much light on geographical distribution
that I am strongly inclined to trust in it; but I will first give the facts
which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the Organ Mountains of Brazil some few temperate European, some Antarctic
and some Andean genera were found by Gardner which do not exist in the low
intervening hot countries. On the Silla of Caraccas the illustrious
Humboldt long ago found species belonging to genera characteristic of the
Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains
several representative European forms are found which have not been
discovered in the intertropical parts of Africa. Dr. Hooker has also
lately shown that several of the plants living on the upper parts of the
lofty island of Fernando Po, and on the neighbouring Cameroon Mountains, in
the Gulf of Guinea, are closely related to those on the mountains of
Abyssinia, and likewise to those of temperate Europe. It now also appears,
as I hear from Dr. Hooker, that some of these same temperate plants have
been discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde
Islands. This extension of the same temperate forms, almost under the
equator, across the whole continent of Africa and to the mountains of the
Cape Verde archipelago, is one of the most astonishing facts ever recorded
in the distribution of plants.
On the Himalaya, and on the isolated mountain ranges of the peninsula of
India, on the heights of Ceylon, and on the volcanic cones of Java, many
plants occur either identically the same or representing each other, and at
the same time representing plants of Europe not found in the intervening
hot lowlands. A list of the genera of plants collected on the loftier
peaks of Java, raises a picture of a collection made on a hillock in
Europe. Still more striking is the fact that peculiar Australian forms are
represented by certain plants growing on the summits of the mountains of
Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend
along the heights of the peninsula of Malacca, and are thinly scattered on
the one hand over India, and on the other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
of European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable "Introduction to the Flora of New Zealand," by
Dr. Hooker, analogous and striking facts are given in regard to the plants
of that large island. Hence, we see that certain plants growing on the
more lofty mountains of the tropics in all parts of the world, and on the
temperate plains of the north and south, are either the same species or
varieties of the same species. It should, however, be observed that these
plants are not strictly arctic forms; for, as Mr. H.C. Watson has remarked,
"in receding from polar toward equatorial latitudes, the Alpine or mountain
flora really become less and less Arctic." Besides these identical and
closely allied forms, many species inhabiting the same widely sundered
areas, belong to genera not now found in the intermediate tropical
lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by the
highest authority, Prof. Dana, that "it is certainly a wonderful fact that
New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.
>From the foregoing facts, namely, the presence of temperate forms on the
highlands across the whole of equatorial Africa, and along the Peninsula of
India, to Ceylon and the Malay Archipelago, and in a less well-marked
manner across the wide expanse of tropical South America, it appears almost
certain that at some former period, no doubt during the most severe part of
a Glacial period, the lowlands of these great continents were everywhere
tenanted under the equator by a considerable number of temperate forms. At
this period the equatorial climate at the level of the sea was probably
about the same with that now experienced at the height of from five to six
thousand feet under the same latitude, or perhaps even rather cooler.
During this, the coldest period, the lowlands under the equator must have
been clothed with a mingled tropical and temperate vegetation, like that
described by Hooker as growing luxuriantly at the height of from four to
five thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of about five
thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with
forms of the torrid zone harmoniously blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
The Glacial period, as measured by years, must have been very long; and
when we remember over what vast spaces some naturalised plants and animals
have spread within a few centuries, this period will have been ample for
any amount of migration. As the cold became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just
given, there can hardly be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equatorial lowlands. The
inhabitants of these hot lowlands would at the same time have migrated to
the tropical and subtropical regions of the south, for the southern
hemisphere was at this period warmer. On the decline of the Glacial
period, as both hemispheres gradually recovered their former temperature,
the northern temperate forms living on the lowlands under the equator,
would have been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some, however,
of the northern temperate forms would almost certainly have ascended any
adjoining high land, where, if sufficiently lofty, they would have long
survived like the Arctic forms on the mountains of Europe. They might have
survived, even if the climate was not perfectly fitted for them, for the
change of temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimatisation, as shown by their
transmitting to their offspring different constitutional powers of
resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These
latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with them
some of the northern temperate forms which had descended from their
mountain fastnesses. Thus, we should have some few species identically the
same in the northern and southern temperate zones and on the mountains of
the intermediate tropical regions. But the species left during a long time
on these mountains, or in opposite hemispheres, would have to compete with
many new forms and would be exposed to somewhat different physical
conditions; hence, they would be eminently liable to modification, and
would generally now exist as varieties or as representative species; and
this is the case. We must, also, bear in mind the occurrence in both
hemispheres of former Glacial periods; for these will account, in
accordance with the same principles, for the many quite distinct species
inhabiting the same widely separated areas, and belonging to genera not now
found in the intermediate torrid zones.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that this preponderant
migration from the north to the south is due to the greater extent of land
in the north, and to the northern forms having existed in their own homes
in greater numbers, and having consequently been advanced through natural
selection and competition to a higher stage of perfection, or dominating
power, than the southern forms. And thus, when the two sets became
commingled in the equatorial regions, during the alternations of the
Glacial periods, the northern forms were the more powerful and were able to
hold their places on the mountains, and afterwards migrate southward with
the southern forms; but not so the southern in regard to the northern
forms. In the same manner, at the present day, we see that very many
European productions cover the ground in La Plata, New Zealand, and to a
lesser degree in Australia, and have beaten the natives; whereas extremely
few southern forms have become naturalised in any part of the northern
hemisphere, though hides, wool, and other objects likely to carry seeds
have been largely imported into Europe during the last two or three
centuries from La Plata and during the last forty or fifty years from
Australia. The Neilgherrie Mountains in India, however, offer a partial
exception; for here, as I hear from Dr. Hooker, Australian forms are
rapidly sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to the more
dominant forms generated in the larger areas and more efficient workshops
of the north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalised; and this is
the first stage towards their extinction. Mountains are islands on the
land; and their inhabitants have yielded to those produced within the
larger areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental forms
naturalised through man's agency.
The same principles apply to the distribution of terrestrial animals and of
marine productions, in the northern and southern temperate zones, and on
the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are, some
of the inhabitants of the temperate seas might have reached the equator; of
these a few would perhaps at once be able to migrate southwards, by keeping
to the cooler currents, while others might remain and survive in the colder
depths until the southern hemisphere was in its turn subjected to a glacial
climate and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by Arctic productions exist
to the present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now
live so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
exact lines of migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species have been
modified and have given rise to new forms, while others have remained
unaltered. We cannot hope to explain such facts, until we can say why one
species and not another becomes naturalised by man's agency in a foreign
land; why one species ranges twice or thrice as far, and is twice or thrice
as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance, the
occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs,
as suggested by Lyell, may have been concerned in their dispersal. The
existence at these and other distant points of the southern hemisphere, of
species, which, though distinct, belong to genera exclusively confined to
the south, is a more remarkable case. Some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the last Glacial period for their migration and subsequent
modification to the necessary degree. The facts seem to indicate that
distinct species belonging to the same genera have migrated in radiating
lines from a common centre; and I am inclined to look in the southern, as
in the northern hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It may
be suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains a
multitude of facts in the distribution of the same and of the allied forms
of life in all parts of the globe. The living waters have flowed during
one period from the north and during another from the south, and in both
cases have reached the equator; but the stream of life has flowed with
greater force from the north than in the opposite direction, and has
consequently more freely inundated the south. As the tide leaves its drift
in horizontal lines, rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our mountain
summits, in a line gently rising from the Arctic lowlands to a great
latitude under the equator. The various beings thus left stranded may be
compared with savage races of man, driven up and surviving in the mountain
fastnesses of almost every land, which serves as a record, full of interest
to us, of the former inhabitants of the surrounding lowlands.
CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION--continued.
Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapters.
FRESH-WATER PRODUCTIONS.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. When first collecting in the fresh waters of Brazil, I well
remember feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh-water productions can, I think, in most
cases be explained by their having become fitted, in a manner highly useful
to them, for short and frequent migrations from pond to pond, or from
stream to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed
that the same fresh-water species never existed on two continents distant
from each other. But Dr. Gunther has lately shown that the Galaxias
attenuatus inhabits Tasmania, New Zealand, the Falkland Islands and the
mainland of South America. This is a wonderful case, and probably
indicates dispersal from an Antarctic centre during a former warm period.
This case, however, is rendered in some degree less surprising by the
species of this genus having the power of crossing by some unknown means
considerable spaces of open ocean: thus there is one species common to New
Zealand and to the Auckland Islands, though separated by a distance of
about 230 miles. On the same continent fresh-water fish often range
widely, and as if capriciously; for in two adjoining river systems some of
the species may be the same and some wholly different.
It is probable that they are occasionally transported by what may be called
accidental means. Thus fishes still alive are not very rarely dropped at
distant points by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the water. Their
dispersal may, however, be mainly attributed to changes in the level of the
land within the recent period, causing rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. The wide differences of the fish on the
opposite sides of most mountain-ranges, which are continuous and
consequently must, from an early period, have completely prevented the
inosculation of the river systems on the two sides, leads to the same
conclusion. Some fresh-water fish belong to very ancient forms, and in
such cases there will have been ample time for great geographical changes,
and consequently time and means for much migration. Moreover, Dr. Gunther
has recently been led by several considerations to infer that with fishes
the same forms have a long endurance. Salt-water fish can with care be
slowly accustomed to live in fresh water; and, according to Valenciennes,
there is hardly a single group of which all the members are confined to
fresh water, so that a marine species belonging to a fresh-water group
might travel far along the shores of the sea, and could, it is probable,
become adapted without much difficulty to the fresh waters of a distant
land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalised
species have spread rapidly throughout the same country. But two facts,
which I have observed--and many others no doubt will be discovered--throw
some light on this subject. When ducks suddenly emerge from a pond covered
with duck-weed, I have twice seen these little plants adhering to their
backs; and it has happened to me, in removing a little duck-weed from one
aquarium to another, that I have unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova
of fresh-water shells were hatching; and I found that numbers of the
extremely minute and just-hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred
off, though at a somewhat more advanced age they would voluntarily drop
off. These just-hatched molluscs, though aquatic in their nature, survived
on the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any other
distant point, would be sure to alight on a pool or rivulet. Sir Charles
Lyell informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a water-beetle
of the same family, a Colymbetes, once flew on board the "Beagle," when
forty-five miles distant from the nearest land: how much farther it might
have been blown by a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges many
fresh-water, and even marsh-species, have, both over continents and to the
most remote oceanic islands. This is strikingly illustrated, according to
Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally adheres
in some quantity to the feet and beaks of birds. Wading birds, which
frequent the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order wander more than those of
any other; and are occasionally found on the most remote and barren islands
of the open ocean; they would not be likely to alight on the surface of the
sea, so that any dirt on their feet would not be washed off; and when
gaining the land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud of
ponds is with seeds: I have tried several little experiments, but will
here give only the most striking case: I took in February three
tablespoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; I
kept it covered up in my study for six months, pulling up and counting each
plant as it grew; the plants were of many kinds, and were altogether 537 in
number; and yet the viscid mud was all contained in a breakfast cup!
Considering these facts, I think it would be an inexplicable circumstance
if water-birds did not transport the seeds of fresh-water plants to
unstocked ponds and streams, situated at very distant points. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected
many hours afterwards in pellets or in the excrement. When I saw the great
size of the seeds of that fine water-lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on the distribution of this plant, I thought
that the means of its dispersal must remain inexplicable; but Audubon
states that he found the seeds of the great southern water-lily (probably
according to Dr. Hooker, the Nelumbium luteum) in a heron's stomach. Now
this bird must often have flown with its stomach thus well stocked to
distant ponds, and, then getting a hearty meal of fish, analogy makes me
believe that it would have rejected the seeds in the pellet in a fit state
for germination.
In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance on a rising islet,
it will be unoccupied; and a single seed or egg will have a good chance of
succeeding. Although there will always be a struggle for life between the
inhabitants of the same pond, however few in kind, yet as the number even
in a well-stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will
probably be less severe than between terrestrial species; consequently an
intruder from the waters of a foreign country would have a better chance of
seizing on a new place, than in the case of terrestrial colonists. We
should also remember that many fresh-water productions are low in the scale
of nature, and we have reason to believe that such beings become modified
more slowly than the high; and this will give time for the migration of
aquatic species. We should not forget the probability of many fresh-water
forms having formerly ranged continuously over immense areas, and then
having become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the same
identical form, or in some degree modified, apparently depends in main part
on the wide dispersal of their seeds and eggs by animals, more especially
by fresh-water birds, which have great powers of flight, and naturally
travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same
species have migrated from some one area, but that allied species, although
now inhabiting the most distant points, have proceeded from a single area,
the birthplace of their early progenitors. I have already given my reasons
for disbelieving in continental extensions within the period of existing
species on so enormous a scale that all the many islands of the several
oceans were thus stocked with their present terrestrial inhabitants. This
view removes many difficulties, but it does not accord with all the facts
in regard to the productions of islands. In the following remarks I shall
not confine myself to the mere question of dispersal, but shall consider
some other cases bearing on the truth of the two theories of independent
creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780 miles
of latitude, together with the outlying islands of Auckland, Campbell and
Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal areas
in Southwestern Australia or at the Cape of Good Hope, we must admit that
some cause, independently of different physical conditions, has given rise
to so great a difference in number. Even the uniform county of Cambridge
has 847 plants, and the little island of Anglesea 764, but a few ferns and
a few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the barren
island of Ascension aboriginally possessed less than half-a-dozen flowering
plants; yet many species have now become naturalised on it, as they have in
New Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and animals
have nearly or quite exterminated many native productions. He who admits
the doctrine of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and animals were not
created for oceanic islands; for man has unintentionally stocked them far
more fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the proportion
of endemic kinds (i.e. those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic
land-shells in Madeira, or of endemic birds in the Galapagos Archipelago,
with the number found on any continent, and then compare the area of the
island with that of the continent, we shall see that this is true. This
fact might have been theoretically expected, for, as already explained,
species occasionally arriving, after long intervals of time in the new and
isolated district, and having to compete with new associates, would be
eminently liable to modification, and would often produce groups of
modified descendants. But it by no means follows that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations have not been
much disturbed; and partly on the frequent arrival of unmodified immigrants
from the mother-country, with which the insular forms have intercrossed.
It should be borne in mind that the offspring of such crosses would
certainly gain in vigour; so that even an occasional cross would produce
more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there are
twenty-six land birds; of these twenty-one (or perhaps twenty-three) are
peculiar; whereas of the eleven marine birds only two are peculiar; and it
is obvious that marine birds could arrive at these islands much more easily
and frequently than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess a
single endemic land bird; and we know from Mr. J.M. Jones's admirable
account of Bermuda, that very many North American birds occasionally or
even frequently visit this island. Almost every year, as I am informed by
Mr. E.V. Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three or four
other species are confined to this island and to the Canaries. So that the
islands of Bermuda and Madeira have been stocked from the neighbouring
continents with birds, which for long ages have there struggled together,
and have become mutually co-adapted. Hence, when settled in their new
homes, each kind will have been kept by the others to its proper place and
habits, and will consequently have been but little liable to modification.
Any tendency to modification will also have been checked by intercrossing
with the unmodified immigrants, often arriving from the mother-country.
Madeira again is inhabited by a wonderful number of peculiar land-shells,
whereas not one species of sea-shell is peculiar to its shores: now,
though we do not know how sea-shells are dispersed, yet we can see that
their eggs or larvae, perhaps attached to seaweed or floating timber, or to
the feet of wading birds, might be transported across three or four hundred
miles of open sea far more easily than land-shells. The different orders
of insects inhabiting Madeira present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is here
spoken of as an oceanic island, it is in some degree doubtful whether it
should be so ranked; it is of large size, and is not separated from
Australia by a profoundly deep sea; from its geological character and the
direction of its mountain ranges, the Rev. W.B. Clarke has lately
maintained that this island, as well as New Caledonia, should be considered
as appurtenances of Australia. Turning to plants, Dr. Hooker has shown
that in the Galapagos Islands the proportional numbers of the different
orders are very different from what they are elsewhere. All such
differences in number, and the absence of certain whole groups of animals
and plants, are generally accounted for by supposed differences in the
physical conditions of the islands; but this explanation is not a little
doubtful. Facility of immigration seems to have been fully as important as
the nature of the conditions.
Many remarkable little facts could be given with respect to the inhabitants
of oceanic islands. For instance, in certain islands not tenanted by a
single mammal, some of the endemic plants have beautifully hooked seeds;
yet few relations are more manifest than that hooks serve for the
transportal of seeds in the wool or fur of quadrupeds. But a hooked seed
might be carried to an island by other means; and the plant then becoming
modified would form an endemic species, still retaining its hooks, which
would form a useless appendage, like the shrivelled wings under the
soldered wing-covers of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only herbaceous
species; now trees, as Alph. de Candolle has shown, generally have,
whatever the cause may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands; and an herbaceous plant, which had
no chance of successfully competing with the many fully developed trees
growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to the
stature of the plant, to whatever order it belonged, and thus first convert
it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
are never found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and have found it
true, with the exception of New Zealand, New Caledonia, the Andaman
Islands, and perhaps the Solomon Islands and the Seychelles. But I have
already remarked that it is doubtful whether New Zealand and New Caledonia
ought to be classed as oceanic islands; and this is still more doubtful
with respect to the Andaman and Solomon groups and the Seychelles. This
general absence of frogs, toads and newts on so many true oceanic islands
cannot be accounted for by their physical conditions; indeed it seems that
islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have multiplied so
as to become a nuisance. But as these animals and their spawn are
immediately killed (with the exception, as far as known, of one Indian
species) by sea-water, there would be great difficulty in their transportal
across the sea, and therefore we can see why they do not exist on strictly
oceanic islands. But why, on the theory of creation, they should not have
been created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of a
terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance
of about 280 miles; moreover, icebergs formerly brought boulders to its
western shores, and they may have formerly transported foxes, as now
frequently happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is known that new species
of mammals appear and disappear at a quicker rate than other and lower
animals. Although terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses two
bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may be asked, has
the supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no
terrestrial mammal can be transported across a wide space of sea, but bats
can fly across. Bats have been seen wandering by day far over the Atlantic
Ocean; and two North American species, either regularly or occasionally,
visit Bermuda, at the distance of 600 miles from the mainland. I hear from
Mr. Tomes, who has specially studied this family, that many species have
enormous ranges, and are found on continents and on far distant islands.
Hence, we have only to suppose that such wandering species have been
modified in their new homes in relation to their new position, and we can
understand the presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by Mr.
Wallace's admirable researches, in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean, and this
separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands are
inhabited by the same or by closely allied quadrupeds. I have not as yet
had time to follow up this subject in all quarters of the world; but as far
as I have gone, the relation holds good. For instance, Britain is
separated by a shallow channel from Europe, and the mammals are the same on
both sides; and so it is with all the islands near the shores of Australia.
The West Indian Islands, on the other hand, stand on a deeply submerged
bank, nearly one thousand fathoms in depth, and here we find American
forms, but the species and even the genera are quite distinct. As the
amount of modification which animals of all kinds undergo partly depends on
the lapse of time, and as the islands which are separated from each other,
or from the mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands separated by
deeper channels, we can understand how it is that a relation exists between
the depth of the sea separating two mammalian faunas, and the degree of
their affinity, a relation which is quite inexplicable on the theory of
independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic islands,
namely, the fewness of the species, with a large proportion consisting of
endemic forms--the members of certain groups, but not those of other groups
in the same class, having been modified--the absence of certain whole
orders, as of batrachians and of terrestrial mammals, notwithstanding the
presence of aerial bats, the singular proportions of certain orders of
plants, herbaceous forms having been developed into trees, etc., seem to me
to accord better with the belief in the efficiency of occasional means of
transport, carried on during a long course of time, than with the belief in
the former connection of all oceanic islands with the nearest continent;
for on this latter view it is probable that the various classes would have
immigrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and
consequently, they would either have not been modified, or all the species
in a more equable manner.
I do not deny that there are many and serious difficulties in understanding
how many of the inhabitants of the more remote islands, whether still
retaining the same specific form or subsequently modified, have reached
their present homes. But the probability of other islands having once
existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will specify one difficult case. Almost all oceanic
islands, even the most isolated and smallest, are inhabited by land-shells,
generally by endemic species, but sometimes by species found elsewhere
striking instances of which have been given by Dr. A.A. Gould in relation
to the Pacific. Now it is notorious that land-shells are easily killed by
sea-water; their eggs, at least such as I have tried, sink in it and are
killed. Yet there must be some unknown, but occasionally efficient means
for their transportal. Would the just-hatched young sometimes adhere to
the feet of birds roosting on the ground and thus get transported? It
occurred to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I find that
several species in this state withstand uninjured an immersion in sea-water
during seven days. One shell, the Helix pomatia, after having been thus
treated, and again hybernating, was put into sea-water for twenty days and
perfectly recovered. During this length of time the shell might have been
carried by a marine country of average swiftness to a distance of 660
geographical miles. As this Helix has a thick calcareous operculum I
removed it, and when it had formed a new membranous one, I again immersed
it for fourteen days in sea-water, and again it recovered and crawled away.
Baron Aucapitaine has since tried similar experiments. He placed 100 land-
shells, belonging to ten species, in a box pierced with holes, and immersed
it for a fortnight in the sea. Out of the hundred shells twenty-seven
recovered. The presence of an operculum seems to have been of importance,
as out of twelve specimens of Cyclostoma elegans, which is thus furnished,
eleven revived. It is remarkable, seeing how well the Helix pomatia
resisted with me the salt-water, that not one of fifty-four specimens
belonging to four other species of Helix tried by Aucapitaine recovered.
It is, however, not at all probable that land-shells have often been thus
transported; the feet of birds offer a more probable method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.
The most striking and important fact for us is the affinity of the species
which inhabit islands to those of the nearest mainland, without being
actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a distance of between 500
and 600 miles from the shores of South America. Here almost every product
of the land and of the water bears the unmistakable stamp of the American
continent. There are twenty-six land birds. Of these twenty-one, or
perhaps twenty-three, are ranked as distinct species, and would commonly be
assumed to have been here created; yet the close affinity of most of these
birds to American species is manifest in every character in their habits,
gestures, and tones of voice. So it is with the other animals, and with a
large proportion of the plants, as shown by Dr. Hooker in his admirable
Flora of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the Pacific, distant several hundred miles from
the continent, feels that he is standing on American land. Why should this
be so? Why should the species which are supposed to have been created in
the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of
affinity to those created in America? There is nothing in the conditions
of life, in the geological nature of the islands, in their height or
climate, or in the proportions in which the several classes are associated
together, which closely resembles the conditions of the South American
coast. In fact, there is a considerable dissimilarity in all these
respects. On the other hand, there is a considerable degree of resemblance
in the volcanic nature of the soil, in the climate, height, and size of the
islands, between the Galapagos and Cape Verde Archipelagos: but what an
entire and absolute difference in their inhabitants! The inhabitants of
the Cape Verde Islands are related to those of Africa, like those of the
Galapagos to America. Facts, such as these, admit of no sort of
explanation on the ordinary view of independent creation; whereas, on the
view here maintained, it is obvious that the Galapagos Islands would be
likely to receive colonists from America, whether by occasional means of
transport or (though I do not believe in this doctrine) by formerly
continuous land, and the Cape Verde Islands from Africa; such colonists
would be liable to modification--the principle of inheritance still
betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the nearest
continent, or of the nearest large island. The exceptions are few, and
most of them can be explained. Thus, although Kerguelen Land stands nearer
to Africa than to America, the plants are related, and that very closely,
as we know from Dr. Hooker's account, to those of America: but on the view
that this island has been mainly stocked by seeds brought with earth and
stones on icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic plants is much more closely related
to Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enormously
remote, that the fact becomes an anomaly. But this difficulty partially
disappears on the view that New Zealand, South America, and the other
southern lands, have been stocked in part from a nearly intermediate though
distant point, namely, from the antarctic islands, when they were clothed
with vegetation, during a warmer tertiary period, before the commencement
of the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the south-western
corner of Australia and of the Cape of Good Hope, is a far more remarkable
case; but this affinity is confined to the plants, and will, no doubt, some
day be explained.
The same law which has determined the relationship between the inhabitants
of islands and the nearest mainland, is sometimes displayed on a small
scale, but in a most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos Archipelago is
tenanted, and the fact is a marvellous one, by many distinct species; but
these species are related to each other in a very much closer manner than
to the inhabitants of the American continent, or of any other quarter of
the world. This is what might have been expected, for islands situated so
near to each other would almost necessarily receive immigrants from the
same original source, and from each other. But how is it that many of the
immigrants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same geological
nature, the same height, climate, etc? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of
considering the physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the other species with
which each has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the species which
inhabit the Galapagos Archipelago, and are likewise found in other parts of
the world, we find that they differ considerably in the several islands.
This difference might indeed have been expected if the islands have been
stocked by occasional means of transport--a seed, for instance, of one
plant having been brought to one island, and that of another plant to
another island, though all proceeding from the same general source. Hence,
when in former times an immigrant first settled on one of the islands, or
when it subsequently spread from one to another, it would undoubtedly be
exposed to different conditions in the different islands, for it would have
to compete with a different set of organisms; a plant, for instance, would
find the ground best-fitted for it occupied by somewhat different species
in the different islands, and would be exposed to the attacks of somewhat
different enemies. If, then, it varied, natural selection would probably
favour different varieties in the different islands. Some species,
however, might spread and yet retain the same character throughout the
group, just as we see some species spreading widely throughout a continent
and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous cases, is that each new species after
being formed in any one island, did not spread quickly to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and deep between
the islands, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they appear
on a map. Nevertheless, some of the species, both of those found in other
parts of the world and of those confined to the archipelago, are common to
the several islands; and we may infer from the present manner of
distribution that they have spread from one island to the others. But we
often take, I think, an erroneous view of the probability of closely allied
species invading each other's territory, when put into free
intercommunication. Undoubtedly, if one species has any advantage over
another, it will in a very brief time wholly or in part supplant it; but if
both are equally well fitted for their own places, both will probably hold
their separate places for almost any length of time. Being familiar with
the fact that many species, naturalised through man's agency, have spread
with astonishing rapidity over wide areas, we are apt to infer that most
species would thus spread; but we should remember that the species which
become naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to distinct genera. In
the Galapagos Archipelago, many even of the birds, though so well adapted
for flying from island to island, differ on the different islands; thus
there are three closely allied species of mocking-thrush, each confined to
its own island. Now let us suppose the mocking-thrush of Chatham Island to
be blown to Charles Island, which has its own mocking-thrush; why should it
succeed in establishing itself there? We may safely infer that Charles
Island is well stocked with its own species, for annually more eggs are
laid and young birds hatched than can possibly be reared; and we may infer
that the mocking-thrush peculiar to Charles Island is at least as well
fitted for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact bearing
on this subject; namely, that Madeira and the adjoining islet of Porto
Santo possess many distinct but representative species of land-shells, some
of which live in crevices of stone; and although large quantities of stone
are annually transported from Porto Santo to Madeira, yet this latter
island has not become colonised by the Porto Santo species: nevertheless,
both islands have been colonised by some European land-shells, which no
doubt had some advantage over the indigenous species. From these
considerations I think we need not greatly marvel at the endemic species
which inhabit the several islands of the Galapagos Archipelago not having
all spread from island to island. On the same continent, also,
pre-occupation has probably played an important part in checking the
commingling of the species which inhabit different districts with nearly
the same physical conditions. Thus, the south-east and south-west corners
of Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants; so it is, according to Mr. Bates, with the
butterflies and other animals inhabiting the great, open, and continuous
valley of the Amazons.
The same principle which governs the general character of the inhabitants
of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent
modification, is of the widest application throughout nature. We see this
on every mountain-summit, in every lake and marsh. For Alpine species,
excepting in as far as the same species have become widely spread during
the Glacial epoch, are related to those of the surrounding lowlands; thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all strictly belonging to American forms; and it is obvious
that a mountain, as it became slowly upheaved, would be colonised from the
surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has allowed the same
forms to prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. It
will, I believe, be found universally true, that wherever in two regions,
let them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species; and wherever
many closely-allied species occur, there will be found many forms which
some naturalists rank as distinct species, and others as mere varieties;
these doubtful forms showing us the steps in the process of modification.
The relation between the power and extent of migration in certain species,
either at the present or at some former period, and the existence at remote
points of the world of closely allied species, is shown in another and more
general way. Mr. Gould remarked to me long ago, that in those genera of
birds which range over the world, many of the species have very wide
ranges. I can hardly doubt that this rule is generally true, though
difficult of proof. Among mammals, we see it strikingly displayed in Bats,
and in a lesser degree in the Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the
inhabitants of fresh water, for many of the genera in the most distinct
classes range over the world, and many of the species have enormous ranges.
It is not meant that all, but that some of the species have very wide
ranges in the genera which range very widely. Nor is it meant that the
species in such genera have, on an average, a very wide range; for this
will largely depend on how far the process of modification has gone; for
instance, two varieties of the same species inhabit America and Europe, and
thus the species has an immense range; but, if variation were to be carried
a little further, the two varieties would be ranked as distinct species,
and their range would be greatly reduced. Still less is it meant, that
species which have the capacity of crossing barriers and ranging widely, as
in the case of certain powerfully-winged birds, will necessarily range
widely; for we should never forget that to range widely implies not only
the power of crossing barriers, but the more important power of being
victorious in distant lands in the struggle for life with foreign
associates. But according to the view that all the species of a genus,
though distributed to the most remote points of the world, are descended
from a single progenitor, we ought to find, and I believe as a general rule
we do find, that some at least of the species range very widely.
We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for dispersal
and subsequent modification. There is also reason to believe, from
geological evidence, that within each great class the lower organisms
change at a slower rate than the higher; consequently they will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with that of the seeds and eggs of most
lowly organised forms being very minute and better fitted for distant
transportal, probably accounts for a law which has long been observed, and
which has lately been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the more widely it
ranges.
The relations just discussed--namely, lower organisms ranging more widely
than the higher--some of the species of widely-ranging genera themselves
ranging widely--such facts, as alpine, lacustrine, and marsh productions
being generally related to those which live on the surrounding low lands
and dry lands--the striking relationship between the inhabitants of islands
and those of the nearest mainland--the still closer relationship of the
distinct inhabitants of the islands of the same archipelago--are
inexplicable on the ordinary view of the independent creation of each
species, but are explicable if we admit colonisation from the nearest or
readiest source, together with the subsequent adaptation of the colonists
to their new homes.
SUMMARY OF THE LAST AND PRESENT CHAPTERS.
In these chapters I have endeavoured to show that if we make due allowance
for our ignorance of the full effects of changes of climate and of the
level of the land, which have certainly occurred within the recent period,
and of other changes which have probably occurred--if we remember how
ignorant we are with respect to the many curious means of occasional
transport--if we bear in mind, and this is a very important consideration,
how often a species may have ranged continuously over a wide area, and then
have become extinct in the intermediate tracts--the difficulty is not
insuperable in believing that all the individuals of the same species,
wherever found, are descended from common parents. And we are led to this
conclusion, which has been arrived at by many naturalists under the
designation of single centres of creation, by various general
considerations, more especially from the importance of barriers of all
kinds, and from the analogical distribution of subgenera, genera, and
families.
With respect to distinct species belonging to the same genus, which on our
theory have spread from one parent-source; if we make the same allowances
as before for our ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been thus granted for
their migration, the difficulties are far from insuperable; though in this
case, as in that of the individuals of the same species, they are often
great.
As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course
of time all the individuals of the same species, and likewise of the
several species belonging to the same genus, have proceeded from some one
source; then all the grand leading facts of geographical distribution are
explicable on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, in not only
separating but in apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of related species
within the same areas; and how it is that under different latitudes, for
instance, in South America, the inhabitants of the plains and mountains, of
the forests, marshes, and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since the colonists entered one of the regions, or
both; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or less
direct competition with each other and with the aborigines; and according
as the immigrants were capable of varying more or less rapidly, there would
ensue in the to or more regions, independently of their physical
conditions, infinitely diversified conditions of life; there would be an
almost endless amount of organic action and reaction, and we should find
some groups of beings greatly, and some only slightly modified; some
developed in great force, some existing in scanty numbers--and this we do
find in the several great geographical provinces of the world.
On these same principles we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but that of these, a large
proportion should be endemic or peculiar; and why, in relation to the means
of migration, one group of beings should have all its species peculiar, and
another group, even within the same class, should have all its species the
same with those in an adjoining quarter of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands should
possess their own peculiar species of aerial mammals or bats. We can see
why, in islands, there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the sea
between such islands and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the several
islets, should be closely related to each other, and should likewise be
related, but less closely, to those of the nearest continent, or other
source whence immigrants might have been derived. We can see why, if there
exist very closely allied or representative species in two areas, however
distant from each other, some identical species will almost always there be
found.
As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space; the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the apparent exceptions to the rule are so few that
they may fairly be attributed to our not having as yet discovered in an
intermediate deposit certain forms which are absent in it, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under different
circumstances, or through occasional means of transport, or by the species
having become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum development.
Groups of species, living during the same period of time, or living within
the same area, are often characterised by trifling features in common, as
of sculpture or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in another
class, or only in a different section of the same order, differ greatly
from each other. In both time and space the lowly organised members of
each class generally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to our theory,
these several relations throughout time and space are intelligible; for
whether we look to the allied forms of life which have changed during
successive ages, or to those which have changed after having migrated into
distant quarters, in both cases they are connected by the same bond of
ordinary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means of natural
selection.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY --
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
organs; their origin explained -- Summary.
CLASSIFICATION.
>From the most remote period in the history of the world organic beings have
been found to resemble each other in descending degrees, so that they can
be classed in groups under groups. This classification is not arbitrary
like the grouping of the stars in constellations. The existence of groups
would have been of simple significance, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different,
for it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation and on
Natural Selection, I have attempted to show that within each country it is
the widely ranging, the much diffused and common, that is the dominant
species, belonging to the larger genera in each class, which vary most.
The varieties, or incipient species, thus produced, ultimately become
converted into new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant species. Consequently
the groups which are now large, and which generally include many dominant
species, tend to go on increasing in size. I further attempted to show
that from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, they
constantly tend to diverge in character. This latter conclusion is
supported by observing the great diversity of forms, which, in any small
area, come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the forms which
are increasing in number and diverging in character, to supplant and
exterminate the preceding, less divergent and less improved forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is, that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and the whole of the genera along this upper line form
together one class, for all are descended from one ancient parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
subfamily, distinct from that containing the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when grouped
in subfamilies; and they form a family distinct from that containing the
three genera still further to the right hand, which diverged at an earlier
period. And all these genera, descended from (A), form an order distinct
from the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera into
subfamilies, families and orders, all under one great class. The grand
fact of the natural subordination of organic beings in groups under groups,
which, from its familiarity, does not always sufficiently strike us, is in
my judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus arranged.
In this case there is of course no relation to genealogical succession, and
no cause can at present be assigned for their falling into groups. But
with organic beings the case is different, and the view above given accords
with their natural arrangement in group under group; and no other
explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what is
meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions--that is, by one
sentence to give the characters common, for instance, to all mammals, by
another those common to all carnivora, by another those common to the
dog-genus, and then, by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant by
the Natural System; they believe that it reveals the plan of the Creator;
but unless it be specified whether order in time or space, or both, or what
else is meant by the plan of the Creator, it seems to me that nothing is
thus added to our knowledge. Expressions such as that famous one by
Linnaeus, which we often meet with in a more or less concealed form,
namely, that the characters do not make the genus, but that the genus gives
the characters, seem to imply that some deeper bond is included in our
classifications than mere resemblance. I believe that this is the case,
and that community of descent--the one known cause of close similarity in
organic beings--is the bond, which, though observed by various degrees of
modification, is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that
those parts of the structure which determined the habits of life, and the
general place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards
the external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
"adaptive or analogical characters;" but to the consideration of these
resemblances we shall recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with special habits, the
more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, "The generative organs, being those which are
most remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We
are least likely in the modifications of these organs to mistake a merely
adaptive for an essential character." With plants how remarkable it is
that the organs of vegetation, on which their nutrition and life depend,
are of little signification; whereas the organs of reproduction, with their
product the seed and embryo, are of paramount importance! So again, in
formerly discussing certain morphological characters which are not
functionally important, we have seen that they are often of the highest
service in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which acts
only on serviceable characters.
That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this
fact; and it has been fully acknowledged in the writings of almost every
author. It will suffice to quote the highest authority, Robert Brown, who,
in speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as I
apprehend in every natural family, is very unequal, and in some cases seems
to be entirely lost." Again, in another work he says, the genera of the
Connaraceae "differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestivation. Any one of
these characters singly is frequently of more than generic importance,
though here even, when all taken together, they appear insufficient to
separate Cnestis from Connarus." To give an example among insects: in one
great division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much, and
the differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be
given of the varying importance for classification of the same important
organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the position of the rudimentary florets
is of the highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles--the inflection of the angle
of the lower jaw in Marsupials--the manner in which the wings of insects
are folded--mere colour in certain Algae--mere pubescence on parts of the
flower in grasses--the nature of the dermal covering, as hair or feathers,
in the Vertebrata. If the Ornithorhynchus had been covered with feathers
instead of hair, this external and trifling character would have been
considered by naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may
depart from its allies in several characters, both of high physiological
importance, and of almost universal prevalence, and yet leave us in no
doubt where it should be ranked. Hence, also, it has been found that a
classification founded on any single character, however important that may
be, has always failed; for no part of the organisation is invariably
constant. The importance of an aggregate of characters, even when none are
important, alone explains the aphorism enunciated by Linnaeus, namely, that
the characters do not give the genus, but the genus gives the character;
for this seems founded on the appreciation of many trifling points of
resemblance, too slight to be defined. Certain plants, belonging to the
Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, "The greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus
laugh at our classification." When Aspicarpa produced in France, during
several years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type of
the order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained among the Malpighiaceae. This case well
illustrates the spirit of our classifications.
Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining
a group or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connexion can be discovered between them, especial value is set on them.
As in most groups of animals, important organs, such as those for
propelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable
in classification; but in some groups all these, the most important vital
organs, are found to offer characters of quite subordinate value. Thus, as
Fritz Muller has lately remarked, in the same group of crustaceans,
Cypridina is furnished with a heart, while in two closely allied genera,
namely Cypris and Cytherea, there is no such organ; one species of
Cypridina has well-developed branchiae, while another species is destitute
of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification
of course includes all ages. But it is by no means obvious, on the
ordinary view, why the structure of the embryo should be more important for
this purpose than that of the adult, which alone plays its full part in the
economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryological characters are
the most important of all; and this doctrine has very generally been
admitted as true. Nevertheless, their importance has sometimes been
exaggerated, owing to the adaptive characters of larvae not having been
excluded; in order to show this, Fritz Muller arranged, by the aid of such
characters alone, the great class of crustaceans, and the arrangement did
not prove a natural one. But there can be no doubt that embryonic,
excluding larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main divisions of
flowering plants are founded on differences in the embryo--on the number
and position of the cotyledons, and on the mode of development of the
plumule and radicle. We shall immediately see why these characters possess
so high a value in classification, namely, from the natural system being
genealogical in its arrangement.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series,
which have hardly a character in common; yet the species at both ends, from
being plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best
botanists, such as Mr. Bentham and others, have strongly insisted on their
arbitrary value. Instances could be given among plants and insects, of a
group first ranked by practised naturalists as only a genus, and then
raised to the rank of a subfamily or family; and this has been done, not
because further research has detected important structural differences, at
first overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification--that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, all true
classification being genealogical--that community of descent is the hidden
bond which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general propositions, and
the mere putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to each
other, must be strictly genealogical in order to be natural; but that the
AMOUNT of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly,
being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what
is meant, if he will take the trouble to refer to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still earlier
form. In three of these genera (A, F, and I) a species has transmitted
modified descendants to the present day, represented by the fifteen genera
(a14 to z14) on the uppermost horizontal line. Now, all these modified
descendants from a single species are related in blood or descent in the
same degree. They may metaphorically be called cousins to the same
millionth degree, yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species descended from A
be ranked in the same genus with the parent A, or those from I with parent
I. But the existing genus F14 may be supposed to have been but slightly
modified, and it will then rank with the parent genus F; just as some few
still living organisms belong to Silurian genera. So that the comparative
value of the differences between these organic beings, which are all
related to each other in the same degree in blood, has come to be widely
different. Nevertheless, their genealogical ARRANGEMENT remains strictly
true, not only at the present time, but at each successive period of
descent. All the modified descendants from A will have inherited something
in common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants at each successive
stage. If, however, we suppose any descendant of A or of I to have become
so much modified as to have lost all traces of its parentage in this case,
its place in the natural system will be lost, as seems to have occurred
with some few existing organisms. All the descendants of the genus F,
along its whole line of descent, are supposed to have been but little
modified, and they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position. The
representation of the groups as here given in the diagram on a flat
surface, is much too simple. The branches ought to have diverged in all
directions. If the names of the groups had been simply written down in a
linear series the representation would have been still less natural; and it
is notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature among the beings of the same
group. Thus, the natural system is genealogical in its arrangement, like a
pedigree. But the amount of modification which the different groups have
undergone has to be expressed by ranking them under different so-called
genera, subfamilies, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, were to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient languages had altered very
little and had given rise to few new languages, whilst others had altered
much owing to the spreading, isolation and state of civilisation of the
several co-descended races, and had thus given rise to many new dialects
and languages. The various degrees of difference between the languages of
the same stock would have to be expressed by groups subordinate to groups;
but the proper or even the only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect
together all languages, extinct and recent, by the closest affinities, and
would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties under
the varieties; and in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are followed as in
classifying species. Authors have insisted on the necessity of arranging
varieties on a natural instead of an artificial system; we are cautioned,
for instance, not to class two varieties of the pine-apple together, merely
because their fruit, though the most important part, happens to be nearly
identical; no one puts the Swedish and common turnip together, though the
esculent and thickened stems are so similar. Whatever part is found to be
most constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the body, etc.;
whereas with sheep the horns are much less serviceable, because less
constant. In classing varieties, I apprehend that if we had a real
pedigree, a genealogical classification would be universally preferred; and
it has been attempted in some cases. For we might feel sure, whether there
had been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of points.
In tumbler pigeons, though some of the subvarieties differ in the important
character of the length of the beak, yet all are kept together from having
the common habit of tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any thought on the subject, these
tumblers are kept in the same group, because allied in blood and alike in
some other respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of species, the two sexes; and how enormously these sometimes differ in the
most important characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which had
previously been ranked as three distinct genera, were known to be sometimes
produced on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, however much they
may differ from each other and from the adult; as well as the so-called
alternate generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties, not
from their partial resemblance to the parent-form, but because they are
descended from it.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups, all
under the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides which
have been followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by resemblances of
any kind. Therefore, we choose those characters which are the least likely
to have been modified, in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than other parts of the organisation.
We care not how trifling a character may be--let it be the mere inflection
of the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers--if it prevail throughout
many and different species, especially those having very different habits
of life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by inheritance from a common
parent. We may err in this respect in regard to single points of
structure, but when several characters, let them be ever so trifling,
concur throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters have been
inherited from a common ancestor; and we know that such aggregated
characters have especial value in classification.
We can understand why a species or a group of species may depart from its
allies, in several of its most important characteristics, and yet be safely
classed with them. This may be safely done, and is often done, as long as
a sufficient number of characters, let them be ever so unimportant, betrays
the hidden bond of community of descent. Let two forms have not a single
character in common, yet, if these extreme forms are connected together by
a chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of
high physiological importance--those which serve to preserve life under the
most diverse conditions of existence--are generally the most constant, we
attach especial value to them; but if these same organs, in another group
or section of a group, are found to differ much, we at once value them less
in our classification. We shall presently see why embryological characters
are of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance
between a mouse and a shrew-mouse (Sorex), which belong to different
orders; and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Antechinus) of Australia.
These latter resemblances may be accounted for, as it seems to me, by
adaptation for similarly active movements through thickets and herbage,
together with concealment from enemies.
Among insects there are innumerable instances; thus Linnaeus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the view of characters being of real importance for classification, only
in so far as they reveal descent, we can clearly understand why analogical
or adaptive characters, although of the utmost importance to the welfare of
the being, are almost valueless to the systematist. For animals, belonging
to two most distinct lines of descent, may have become adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal--will rather tend to conceal their
blood-relationship. We can thus also understand the apparent paradox, that
the very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but between the the several
members of the whale family, the shape of the body and the fin-like limbs
offer characters exhibiting true affinity; for as these parts are so nearly
similar throughout the whole family, we cannot doubt that they have been
inherited from a common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite distinct
beings between single parts or organs, which have been adapted for the same
functions. A good instance is afforded by the close resemblance of the
jaws of the dog and Tasmanian wolf or Thylacinus--animals which are widely
sundered in the natural system. But this resemblance is confined to
general appearance, as in the prominence of the canines, and in the cutting
shape of the molar teeth. For the teeth really differ much: thus the dog
has on each side of the upper jaw four pre-molars and only two molars;
while the Thylacinus has three pre-molars and four molars. The molars also
differ much in the two animals in relative size and structure. The adult
dentition is preceded by a widely different milk dentition. Any one may,
of course, deny that the teeth in either case have been adapted for tearing
flesh, through the natural selection of successive variations; but if this
be admitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs--of widely different insects possessing
luminous organs--and of orchids and asclepiads having pollen-masses with
viscid discs, come under this same head of analogical resemblances. But
these cases are so wonderful that they were introduced as difficulties or
objections to our theory. In all such cases some fundamental difference in
the growth or development of the parts, and generally in their matured
structure, can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are essentially different.
The principle formerly alluded to under the term of ANALOGICAL VARIATION
has probably in these cases often come into play; that is, the members of
the same class, although only distantly allied, have inherited so much in
common in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in the
acquirement through natural selection of parts or organs, strikingly like
each other, independently of their direct inheritance from a common
progenitor.
As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances
--to inhabit, for instance, the three elements of land, air and water--we
can perhaps understand how it is that a numerical parallelism has sometimes
been observed between the subgroups of distinct classes. A naturalist,
struck with a parallelism of this nature, by arbitrarily raising or sinking
the value of the groups in several classes (and all our experience shows
that their valuation is as yet arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary, quaternary
and ternary classifications have probably arisen.
There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life, but
has been gained for the sake of protection. I allude to the wonderful
manner in which certain butterflies imitate, as first described by Mr.
Bates, other and quite distinct species. This excellent observer has shown
that in some districts of South America, where, for instance, an Ithomia
abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often
found mingled in the same flock; and the latter so closely resembles the
Ithomia in every shade and stripe of colour, and even in the shape of its
wings, that Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be very
different in essential structure, and to belong not only to distinct
genera, but often to distinct families. Had this mimicry occurred in only
one or two instances, it might have been passed over as a strange
coincidence. But, if we proceed from a district where one Leptalis
imitates an Ithomia, another mocking and mocked species, belonging to the
same two genera, equally close in their resemblance, may be found.
Altogether no less than ten genera are enumerated, which include species
that imitate other butterflies. The mockers and mocked always inhabit the
same region; we never find an imitator living remote from the form which it
imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place, species
of three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves especial
notice that many of the mimicking forms of the Leptalis, as well as of the
mimicked forms, can be shown by a graduated series to be merely varieties
of the same species; while others are undoubtedly distinct species. But
why, it may be asked, are certain forms treated as the mimicked and others
as the mimickers? Mr. Bates satisfactorily answers this question by
showing that the form which is imitated keeps the usual dress of the group
to which it belongs, while the counterfeiters have changed their dress and
do not resemble their nearest allies.
We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit on
the true explanation. The mocked forms, which always abound in numbers,
must habitually escape destruction to a large extent, otherwise they could
not exist in such swarms; and a large amount of evidence has now been
collected, showing that they are distasteful to birds and other insect-
devouring animals. The mocking forms, on the other hand, that inhabit the
same district, are comparatively rare, and belong to rare groups; hence,
they must suffer habitually from some danger, for otherwise, from the
number of eggs laid by all butterflies, they would in three or four
generations swarm over the whole country. Now if a member of one of these
persecuted and rare groups were to assume a dress so like that of a well-
protected species that it continually deceived the practised eyes of an
entomologist, it would often deceive predaceous birds and insects, and thus
often escape destruction. Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In one
district several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much commoner
than the others, and this closely mocked another form of Ithomia. From
facts of this nature, Mr. Bates concludes that the Leptalis first varies;
and when a variety happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of escaping
destruction from predaceous birds and insects, and is consequently oftener
preserved; "the less perfect degrees of resemblance being generation after
generation eliminated, and only the others left to propagate their kind."
So that here we have an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally striking
cases of imitation in the Lepidoptera of the Malay Archipelago and Africa,
and with some other insects. Mr. Wallace has also detected one such case
with birds, but we have none with the larger quadrupeds. The much greater
frequency of imitation with insects than with other animals, is probably
the consequence of their small size; insects cannot defend themselves,
excepting indeed the kinds furnished with a sting, and I have never heard
of an instance of such kinds mocking other insects, though they are mocked;
insects cannot easily escape by flight from the larger animals which prey
on them; therefore, speaking metaphorically, they are reduced, like most
weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the imitated
form was subsequently and gradually modified through any agency, the
imitating form would be led along the same track, and thus be altered to
almost any extent, so that it might ultimately assume an appearance or
colouring wholly unlike that of the other members of the family to which it
belonged. There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members belonging to
several distinct groups, before they had diverged to their present extent,
accidentally resembled a member of another and protected group in a
sufficient degree to afford some slight protection, this having given the
basis for the subsequent acquisition of the most perfect resemblance.
ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups within each class thus tend to go on
increasing in size, and they consequently supplant many smaller and feebler
groups. Thus, we can account for the fact that all organisms, recent and
extinct, are included under a few great orders and under still fewer
classes. As showing how few the higher groups are in number, and how
widely they are spread throughout the world, the fact is striking that the
discovery of Australia has not added an insect belonging to a new class,
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three families of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some degree intermediate between
existing groups. As some few of the old and intermediate forms having
transmitted to the present day descendants but little modified, these
constitute our so-called osculant or aberrant groups. The more aberrant
any form is, the greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence of aberrant
groups having suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example, would not have
been less aberrant had each been represented by a dozen species, instead of
as at present by a single one, or by two or three. We can, I think,
account for this fact only by looking at aberrant groups as forms which
have been conquered by more successful competitors, with a few members
still preserved under unusually favourable conditions.
Mr. Waterhouse has remarked that when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
that is, not to any one Marsupial species more than to another. As these
points of affinity are believed to be real and not merely adaptive, they
must be due in accordance with our view to inheritance from a common
progenitor. Therefore, we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial, which will
naturally have been more or less intermediate in character with respect to
all existing Marsupials; or that both Rodents and Marsupials branched off
from a common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we must suppose that
the bizcacha has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or
nearly all Marsupials, from having partially retained the character of
their common progenitor, or of some early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected that
the resemblance is only analogical, owing to the Phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of
distinct families of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common progenitor, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
progenitor of a whole family, now broken up by extinction into distinct
groups and subgroups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to do
so without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram,
the various affinities which they perceive between the many living and
extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other--for instance, of birds from all other vertebrate animals--by
the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the early
progenitors of the other and at that time less differentiated vertebrate
classes. There has been much less extinction of the forms of life which
once connected fishes with Batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here the most
wonderfully diverse forms are still linked together by a long and only
partially broken chain of affinities. Extinction has only defined the
groups: it has by no means made them; for if every form which has ever
lived on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be distinguished,
still a natural classification, or at least a natural arrangement, would be
possible. We shall see this by turning to the diagram: the letters, A to
L, may represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural; for,
on the principle of inheritance, all the forms descended, for instance from
A, would have something in common. In a tree we can distinguish this or
that branch, though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups; but we could pick
out types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were
ever to succeed in collecting all the forms in any one class which have
lived throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we are
tending toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction and
divergence of character in the descendants from any one parent-species,
explains that great and universal feature in the affinities of all organic
beings, namely, their subordination in group under group. We use the
element of descent in classing the individuals of both sexes and of all
ages under one species, although they may have but few characters in
common; we use descent in classing acknowledged varieties, however
different they may be from their parents; and I believe that this element
of descent is the hidden bond of connexion which naturalists have sought
under the term of the Natural System. On this idea of the natural system
being, in so far as it has been perfected, genealogical in its arrangement,
with the grades of difference expressed by the terms genera, families,
orders, etc., we can understand the rules which we are compelled to follow
in our classification. We can understand why we value certain resemblances
far more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations between
one group and another, we summarily reject analogical or adaptive
characters, and yet use these same characters within the limits of the same
group. We can clearly see how it is that all living and extinct forms can
be grouped together within a few great classes; and how the several members
of each class are connected together by the most complex and radiating
lines of affinities. We shall never, probably, disentangle the
inextricable web of the affinities between the members of any one class;
but when we have a distinct object in view, and do not look to some unknown
plan of creation, we may hope to make sure but slow progress.
Professor Haeckel in his "Generelle Morphologie" and in another works, has
recently brought his great knowledge and abilities to bear on what he calls
phylogeny, or the lines of descent of all organic beings. In drawing up
the several series he trusts chiefly to embryological characters, but
receives aid from homologous and rudimentary organs, as well as from the
successive periods at which the various forms of life are believed to have
first appeared in our geological formations. He has thus boldly made a
great beginning, and shows us how classification will in the future be
treated.
MORPHOLOGY.
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very soul.
What can be more curious than that the hand of a man, formed for grasping,
that of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include similar bones, in the same relative positions?
How curious it is, to give a subordinate though striking instance, that the
hind feet of the kangaroo, which are so well fitted for bounding over the
open plains--those of the climbing, leaf-eating koala, equally well fitted
for grasping the branches of trees--those of the ground-dwelling, insect or
root-eating, bandicoots--and those of some other Australian marsupials--
should all be constructed on the same extraordinary type, namely with the
bones of the second and third digits extremely slender and enveloped within
the same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly the same habits of
life as some of their Australian relatives, having feet constructed on the
ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion: "We may call this conformity to type, without
getting much nearer to an explanation of the phenomenon;" and he then adds
"but is it not powerfully suggestive of true relationship, of inheritance
from a common ancestor?"
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the
arm and forearm, or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
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