Time and Life
by
Thomas H. Huxley
This etext was prepared by Amy E. Zelmer.
TIME AND LIFE*
MR. DARWIN'S "ORIGIN OF SPECIES"
by Thomas H. Huxley
[footnote] *"Macmillan's Magazine", December 1859.
EVERYONE knows that that superficial film of the earth's substance,
hardly ten miles thick, which is accessible to human investigation, is
composed for the most part of beds or strata of stone, the consolidated
muds and sands of former seas and lakes, which have been deposited one
upon the other, and hence are the older the deeper they lie. These
multitudinous strata present such resemblances and differences among
themselves that they are capable of classification into groups or
formations, and these formations again are brigaded together into still
larger assemblages, called by the older geologists, primary, secondary,
and tertiary; by the moderns, palaeozoic, mesozoic, and cainozoic: the
basis of the former nomenclature being the relative age of the groups
of strata; that of the latter, the kinds of living forms contained in
them.
Though but a film if compared with the total diameter of our planet, the
total series of formations is vast indeed when measured by any human
standard, and, as all action implies time, so are we compelled to
regard these mineral masses as a measure of the time which has elapsed
during their accumulation. The amount of the time which they represent
is, of course, in the inverse proportion of the intensity of the forces
which have been in operation. If, in the ancient world, mud and sand
accumulated on sea-bottoms at tenfold their present rate, it is clear
that a bed of mud or sand ten feet thick would have been formed then in
the same time as a stratum of similar materials one foot thick would be
formed now, and 'vice versa'.
At the outset of his studies, therefore, the physical geologist had to
choose between two hypotheses; either, throughout the ages which are
represented by the accumulated strata, and which we may call 'geologic
time', the forces of nature have operated with much same average
intensity as at present, and hence the lapse of time which they
represent must be something prodigious and inconceivable, or, in the
primeval epochs, the natural powers were infinitely more intense than
now, and hence the time through which they acted to produce the effects
we see was comparatively short.
The earlier geologists adopted the latter view almost with one consent.
For they had little knowledge of the present workings of nature, and
they read the records of geologic time as a child reads the history of
Rome or Greece, and fancies that antiquity was grand, heroic, and
unlike the present because it is unlike his little experience of the
present.
Even so the earlier observers were moved with wonder at the seeming
contrast between the ancient and the present order of nature. The
elemental forces seemed to have been grander and more energetic in
primeval times. Upheaved and contorted, rifted and fissured, pierced
by dykes of molten matter or worn away over vast areas by aqueous
action, the older rocks appeared to bear witness to a state of things
far different from that exhibited by the peaceful epoch on which the
lot of man has fallen.
But by degrees thoughtful students of geology have been led to perceive
that the earliest efforts of nature have been by no means the
grandest. Alps and Andes are children of yesterday when compared with
Snowdon and the Cumberland hills; and the so-called glacial epoch--that
in which perhaps the most extensive physical changes of which any
record remaining occurred--is the last and the newest of the
revolutions of the globe. And in proportion as physical
geography--which is the geology of our own epoch--has grown into a
science, and the present order of nature has been ransacked to find
what, 'hibernice', we may call precedents for the phenomena of the
past, so the apparent necessity of supposing the past to be widely
different from the present has diminished.
The transporting power of the greatest deluge which can be imagined
sinks into insignificance beside that of the slowly floating, slowly
melting iceberg, or the glacier creeping along at its snail's pace of a
yard a day. The study of the deltas of the Nile, the Ganges, and the
Mississippi has taught us how slow is the wearing action of water, how
vast its effects when time is allowed for its operation. The reefs of
the Pacific, the deep-sea soundings of the Atlantic, show that it is to
the slow-growing coral and to the imperceptible animalcule, which lives
its brief space and then adds its tiny shell to the muddy cairn left by
its brethren and ancestors, that we must look as the agents in the
formation of limestone and chalk, and not to hypothetical oceans
saturated with calcareous salts and suddenly depositing them.
And while the inquirer has thus learnt that existing forces--'give them
time'--are competent to produce all the physical phenomena we meet with
in the rocks, so, on the other side, the study of the marks left in the
ancient strata by past physical actions shows that these were similar
to those which now obtain. Ancient beaches are met with whose pebbles
are like those found on modern shores; the hardened sea-sands of the
oldest epochs show ripple-marks, such as may now be found on every
sandy coast; nay, more, the pits left by ancient rain-drops prove that
even in the very earliest ages, the "bow in the clouds" must have
adorned the palaeozoic firmament. So that if we could reverse the
legend of the Seven Sleepers,--if we could sleep back through the past,
and awake a million ages before our own epoch, in the midst of the
earliest geologic times,--there is no reason to believe that sea, or
sky, or the aspect of the land would warn us of the marvellous
retrospection.
Such are the beliefs which modern physical geologists hold, or, at any
rate, tend towards holding. But, in so doing, it is obvious that they
by no means prejudge the question, as to what the physical condition of
the globe may have been before our chapters of its history begin, in
what may be called (with that licence which is implied in the often-used
term "prehistoric epoch") "pre-geologic time." The views indicated, in
fact, are not only quite consistent with the hypothesis, that, in the
still earlier period referred to, the condition of our world was very
different; but they may be held by some to necessitate that hypothesis.
The physical philosopher who is accurately acquainted with the velocity
of a cannon-ball, and the precise character of the line which it
traverses for a yard of its course, is necessitated by what he knows of
the laws of nature to conclude that it came from a certain spot, whence
it was impelled by a certain force, and that it has followed a certain
trajectory. In like manner, the student of physical geology, who fully
believes in the uniformity of the general condition of the earth
through geologic time, may feel compelled by what he knows of causation,
and by the general analogy of nature, to suppose that our solar system
was once a nebulous mass; that it gradually condensed, that it broke up
into that wonderful group of harmoniously rolling balls we call planets
and satellites, and that then each of these underwent its appointed
metamorphosis, until at last our own share of the cosmic vapour passed
into that condition in which we first meet with definite records of its
state, and in which it has since, with comparatively little change,
remained.
The doctrine of uniformity and the doctrine of progression are,
therefore, perfectly consistent; perhaps, indeed, they might be shown
to be necessarily connected with one another.
If, however, the condition of the world, which has obtained throughout
geologic time, is but the sequel to a vast series of changes which took
place in pre-geologic time, then it seems not unlikely that the
duration of this latter is to that of the former as the vast extent of
geologic time is to the length of the brief epoch we call the
historical period; and that even the oldest rocks are records of an
epoch almost infinitely remote from that which could have witnessed the
first shaping of our globe.
It is probable that no modern geologist would hesitate to admit the
general validity of these reasonings when applied to the physics of his
subject, whence it is the more remarkable that the moment the question
changes from one of physics and chemistry to one of natural history,
scientific opinions and the popular prejudices, which reflect them in a
distorted form, undergo a sudden metamorphosis. Geologists and
palaeontologists write about the "beginning of life" and the
"first-created forms of living beings," as if they were the most
familiar things in the world; and even cautious writers seem to be on
quite friendly terms with the "archetype" whereby the Creator was
guided "amidst the crash of falling worlds." Just as it used to be
imagined that the ancient world was physically opposed to the present,
so it is still widely assumed that the living population of our globe,
whether animal or vegetable, in the older epochs, exhibited forms so
strikingly contrasted with those which we see around us, that there is
hardly anything in common between the two. It is constantly tacitly
assumed that we have before us all the forms of life which have ever
existed; and though the progress of knowledge, yearly and almost
monthly, drives the defenders of that position from their ground, they
entrench themselves in the new line of defences as if nothing had
happened, and proclaim that the 'new' beginning is the 'real'
beginning.
Without for an instant denying or endeavouring to soften down the
considerable positive differences (the negative ones are met by another
line of argument) which undoubtedly obtain between the ancient and the
modern worlds of life, we believe they have been vastly overstated and
exaggerated, and this belief is based upon certain facts whose value
does not seem to have been fully appreciated, though they have long
been more or less completely known.
The multitudinous kinds of animals and plants, both recent and fossil,
are, as is well known, arranged by zoologists and botanists, in
accordance with their natural relations, into groups which receive the
names of sub-kingdoms, classes, orders, families, genera and species.
Now it is a most remarkable circumstance that, viewed on the great
scale, living beings have differed so little throughout all geologic
time that there is no sub-kingdom and no class wholly extinct or
without living representatives.
If we descend to the smaller groups, we find that the number of orders
of plants is about two hundred; and I have it on the best authority
that not one of these is exclusively fossil; so that there is
absolutely not a single extinct ordinal type of vegetable life; and it
is not until we descend to the next group, or the families, that we
find types which are wholly extinct. The number of orders of animals,
on the other hand, may be reckoned at a hundred and twenty, or
thereabouts, and of these, eight or nine have no living representatives.
The proportion of extinct ordinal types of animals to the existing
types, therefore, does not exceed seven per cent.--a marvellously small
proportion when we consider the vastness of geologic time.
Another class of considerations--of a different kind, it is true, but
tending in the same direction--seems to have been overlooked. Not only
is it true that the general plan of construction of animals and plants
has been the same in all recorded time as at present, but there are
particular kinds of animals and plants which have existed throughout
vast epochs, sometimes through the whole range of recorded time, with
very little change. By reason of this persistency, the typical form of
such a kind might be called a "persistent type," in contradistinction
to those types which have appeared for but a short time in the course of
the world's history. Examples of these persistent types are abundant
enough in both the vegetable and the animal kingdoms. The oldest group
of plants with which we are well acquainted is that of whose remains
coal is constituted; and as far as they can be identified, the
carboniferous plants are ferns, or club-mosses, or Coniferae, in many
cases generically identical with those now living!
Among animals, instances of the same kind may be found in every
sub-kingdom. The 'Globigerina' of the Atlantic soundings is identical
with that which occurs in the chalk; and the casts of lower silurian
'Foraminifera', which Ehrenberg has recently described, seem to
indicate the existence at that remote period of forms singularly like
those which now exist. Among the corals, the palaeozoic 'Tabulata' are
constructed on precisely the same type as the modern millepores; and if
we turn to molluscs, the most competent malacologists fail to discover
any generic distinction between the 'Craniae', 'Lingulae' and
'Discinae' of the silurian rocks and those which now live. Our
existing 'Nautilus' has its representative species in every great
formation, from the oldest to the newest; and 'Loligo', the squid of
modern seas, appears in the lias, or at the bottom of the mesozoic
series, in a form, at most, specifically different from its living
congeners. In the great assemblage of annulose animals, the two highest
classes, the insects and spider tribe, exhibit a wonderful persistency
of type. The cockroaches of the carboniferous epoch are exceedingly
similar to those which now run about our coal-cellars; and its locusts,
termites and dragon-flies are closely allied to the members of the same
groups which now chirrup about our fields, undermine our houses, or
sail with swift grace about the banks of our sedgy pools. And, in like
manner, the palaeozoic scorpions can only be distinguished by the eye
of a naturalist from the modern ones.
Finally, with respect to the 'Vertebrata', the same law holds good:
certain types, such as those of the ganoid and placoid fishes, having
persisted from the palaeozoic epoch to the present time without a
greater amount of deviation from the normal standard than that which is
seen within the limits of the group as it now exists. Even among the
'Reptilia'--the class which exhibits the largest proportion of entirely
extinct forms of any one type,--that of the 'Crocodilia', has persisted
from at least the commencement of the Mesozoic epoch up to the present
time with so much constancy, that the amount of change which it exhibits
may fairly, in relation to the time which has elapsed, be called
insignificant. And the imperfect knowledge we have of the ancient
mammalian population of our earth leads to the belief that certain of
its types, such as that of the 'Marsupialia', have persisted with
correspondingly little change through a similar range of time.
Thus it would appear to be demonstrable, that, notwithstanding the great
change which is exhibited by the animal population of the world as a
whole, certain types have persisted comparatively without alteration,
and the question arises, What bearing have such facts as these on our
notions of the history of life through geological time? The answer to
this question would seem to depend on the view we take respecting the
origin of species in general. If we assume that every species of
animal and of plant was formed by a distinct act of creative power, and
if the species which have incessantly succeeded one another were placed
upon the globe by these separate acts, then the existence of persistent
types is simply an unintelligible irregularity. Such assumption,
however, is as unsupported by tradition or by Revelation as it is
opposed by the analogy of the rest of the operations of nature; and
those who imagine that, by adopting any such hypothesis, they are
strengthening the hands of the advocates of the letter of the Mosaic
account, are simply mistaken. If, on the other hand, we adopt that
hypothesis to which alone the study of physiology lends any
support--that hypothesis which, having struggled beyond the reach of
those fatal supporters, the Telliameds and Vestigiarians, who so nearly
caused its suffocation by wind in early infancy, is now winning at
least the provisional assent of all the best thinkers of the day--the
hypothesis that the forms or species of living beings, as we know them,
have been produced by the gradual modification of pre-existing
species--then the existence of persistent types seems to teach us
much. Just as a small portion of a great curve appears straight, the
apparent absence of change in direction of the line being the exponent
of the vast extent of the whole, in proportion to the part we see; so,
if it be true that all living species are the result of the modification
of other and simpler forms, the existence of these little altered
persistent types, ranging through all geological time, must indicate
that they are but the final terms of an enormous series of
modifications, which had their being in the great lapse of pregeologic
time, and are now perhaps for ever lost.
In other words, when rightly studied, the teachings of palaeontology are
at one with those of physical geology. Our farthest explorations carry
us back but a little way above the mouth of the great river of Life:
where it arose, and by what channels the noble tide has reached the
point when it first breaks upon our view, is hidden from us.
The foregoing pages contain the substance of a lecture delivered before
the Royal Institution of Great Britain many months ago, and of course
long before the appearance of the remarkable work on the "Origin of
Species" just published by Mr. Darwin, who arrives at very similar
conclusions. Although, in one sense, I might fairly say that my own
views have been arrived at independently, I do not know that I can
claim any equitable right to property in them; for it has long been my
privilege to enjoy Mr. Darwin's friendship, and to profit by
corresponding with him, and by, to some extent, becoming acquainted with
the workings of his singularly original and well-stored mind. It was
in consequence of my knowledge of the general tenor of the researches
in which Mr. Darwin had been so long engaged; because I had the most
complete confidence in his perseverance, his knowledge, and, above all
things, his high-minded love of truth; and, moreover, because I found
that the better I became acquainted with the opinions of the best
naturalists regarding the vexed question of species, the less fixed
they seemed to be, and the more inclined they were to the hypothesis of
gradual modification, that I ventured to speak as strongly as I have
done in the final paragraphs of my discourse.
Thus, my daw having so many borrowed plumes, I see no impropriety in
making a tail to this brief paper by taking another handful of feathers
from Mr. Darwin; endeavouring to point out in a few words, in fact,
what, as I gather from the perusal of his book, his doctrines really
are, and on what sort of basis they rest. And I do this the more
willingly, as I observe that already the hastier sort of critics have
begun, not to review my friend's book, but to howl over it in a manner
which must tend greatly to distract the public mind.
No one will be better satisfied than I to see Mr. Darwin's book refuted,
if any person be competent to perform that feat; but I would suggest
that refutation is retarded, not aided, by mere sarcastic
misrepresentation. Every one who has studied cattle-breeding, or
turned pigeon-fancier, or "pomologist," must have been struck by the
extreme modifiability or plasticity of those kinds of animals and
plants which have been subjected to such artificial conditions as are
imposed by domestication. Breeds of dogs are more different from one
another than are the dog and the wolf; and the purely artificial races
of pigeons, if their origin were unknown, would most assuredly be
reckoned by naturalists as distinct species and even genera.
These breeds are always produced in the same way. The breeder selects a
pair, one or other, or both, of which present an indication of the
peculiarity he wishes to perpetuate, and then selects from the
offspring of them those which are most characteristic, rejecting the
others. From the selected offspring he breeds again, and, taking the
same precautions as before, repeats the process until he has obtained
the precise degree of divergence from the primitive type at which he
aimed.
If he now breeds from the variety thus established for some generations,
taking care always to keep the stock pure, the tendency to produce this
particular variety becomes more and more strongly hereditary; and it
does not appear that there is any limit to the persistency of the race
thus developed.
Men like Lamarck, apprehending these facts, and knowing that varieties
comparable to those produced by the breeder are abundantly found in
nature, and finding it impossible to discriminate in some cases between
varieties and true species, could hardly fail to divine the possibility
that species even the most distinct were, after all, only exceedingly
persistent varieties, and that they had arisen by the modification of
some common stock, just as it is with good reason believed that
turnspits and greyhounds, carrier and tumbler pigeons, have arisen.
But there was a link wanting to complete the parallel. Where in nature
was the analogue of the breeder to be found? How could that operation
of selection, which is his essential function, be carried out by mere
natural agencies? Lamarck did not value this problem; neither did he
admit his impotence to solve it; but he guessed a solution. Now,
guessing in science is a very hazardous proceeding, and Lamarck's
reputation has suffered woefully for the absurdities into which his
baseless suppositions led him.
Lamarck's conjectures, equipped with a new hat and stick, as Sir Walter
Scott was wont to say of an old story renovated, formed the foundation
of the biological speculations of the 'Vestiges', a work which has done
more harm to the progress of sound thought on these matters than any
that could be named; and, indeed, I mention it here simply for the
purpose of denying that it has anything in common with what essentially
characterises Mr. Darwin's work.
The peculiar feature of the latter is, in fact, that it professes to
tell us what in nature takes the place of the breeder; what it is that
favours the development of one variety into which a species may run,
and checks that of another; and, finally, shows how this natural
selection, as it is termed, may be the physical cause of the production
of species by modification.
That which takes the place of the breeder and selector in nature is
Death. In a most remarkable chapter, 'On the Struggle for Existence',
Mr. Darwin draws attention to the marvellous destruction of life which
is constantly going on in nature. For every species of living thing,
as for man, "Eine Bresche ist ein jeder Tag."--Every species has its
enemies; every species has to compete with others for the necessaries
of existence; the weakest goes to the wall, and death is the penalty
inflicted on all laggards and stragglers. Every variety to which a
species may give rise is either worse or better adapted to surrounding
circumstances than its parent. If worse, it cannot maintain itself
against death, and speedily vanishes again. But if better adapted, it
must, sooner or later, "improve" its progenitor from the face of the
earth, and take its place. If circumstances change, the victor will be
similarly supplanted by its own progeny; and thus, by the operation of
natural causes, unlimited modification may in the lapse of long ages
occur.
For an explanation of what I have here called vaguely "surrounding
circumstances," and of why they continually change--for ample proof
that the "struggle for existence" is a very great reality, and
assuredly 'tends' to exert the influence ascribed to it--I must refer to
Mr. Darwin's book. I believe I have stated fairly the position upon
which his whole theory must stand or fall; and it is not my purpose to
anticipate a full review of his work. If it can be proved that the
process of natural selection, operating upon any species, can give rise
to varieties of species so different from one another that none of our
tests will distinguish them from true species, Mr. Darwin's hypothesis
of the origin of species will take its place among the established
theories of science, be its consequences whatever they may. If, on the
other hand, Mr. Darwin has erred, either in fact or in reasoning, his
fellow-workers will soon find out the weak points in his doctrines, and
their extinction by some nearer approximation to the truth will
exemplify his own principle of natural selection.
In either case the question is one to be settled only by the
painstaking, truth-loving investigation of skilled naturalists. It is
the duty of the general public to await the result in patience; and,
above all things, to discourage, as they would any other crimes, the
attempt to enlist the prejudices of the ignorant, or the
uncharitableness of the bigoted, on either side of the controversy.
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