Darwin and Modern Science
by
A.C. Seward
Part 2 out of 14
that have since been urged against them. Moreover, the matter is by no
means one of subordinate interest; it is the very kernel of the whole
question of the reality and value of the principle of selection. For if
selection alone does not suffice to explain "HARMONIOUS ADAPTATION" as I
have called Spencer's COADAPTATION, and if we require to call in the aid of
the Lamarckian factor it would be questionable whether selection could
explain any adaptations whatever. In this particular case--of worker bees
--the Lamarckian factor may be excluded altogether, for it can be
demonstrated that here at any rate the effects of use and disuse cannot be
transmitted.
But if it be asked why we are unwilling to admit the cooperation of the
Darwinian factor of selection and the Lamarckian factor, since this would
afford us an easy and satisfactory explanation of the phenomena, I answer:
BECAUSE THE LAMARCKIAN PRINCIPLE IS FALLACIOUS, AND BECAUSE BY ACCEPTING IT
WE CLOSE THE WAY TOWARDS DEEPER INSIGHT. It is not a spirit of
combativeness or a desire for self-vindication that induces me to take the
field once more against the Lamarckian principle, it is the conviction that
the progress of our knowledge is being obstructed by the acceptance of this
fallacious principle, since the facile explanation it apparently affords
prevents our seeking after a truer explanation and a deeper analysis.
The workers in the various species of ants are sterile, that is to say,
they take no regular part in the reproduction of the species, although
individuals among them may occasionally lay eggs. In addition to this they
have lost the wings, and the receptaculum seminis, and their compound eyes
have degenerated to a few facets. How could this last change have come
about through disuse, since the eyes of workers are exposed to light in the
same way as are those of the sexual insects and thus in this particular
case are not liable to "disuse" at all? The same is true of the
receptaculum seminis, which can only have been disused as far as its
glandular portion and its stalk are concerned, and also of the wings, the
nerves tracheae and epidermal cells of which could not cease to function
until the whole wing had degenerated, for the chitinous skeleton of the
wing does not function at all in the active sense.
But, on the other hand, the workers in all species have undergone
modifications in a positive direction, as, for instance, the greater
development of brain. In many species large workers have evolved,--the so-
called SOLDIERS, with enormous jaws and teeth, which defend the colony,--
and in others there are SMALL workers which have taken over other special
functions, such as the rearing of the young Aphides. This kind of division
of the workers into two castes occurs among several tropical species of
ants, but it is also present in the Italian species, Colobopsis truncata.
Beautifully as the size of the jaws could be explained as due to the
increased use made of them by the "soldiers," or the enlarged brain as due
to the mental activities of the workers, the fact of the infertility of
these forms is an insurmountable obstacle to accepting such an explanation.
Neither jaws nor brain can have been evolved on the Lamarckian principle.
The problem of coadaptation is no easier in the case of the ant than in the
case of the Giant Stag. Darwin himself gave a pretty illustration to show
how imposing the difference between the two kinds of workers in one species
would seem if we translated it into human terms. In regard to the Driver
ants (Anomma) we must picture to ourselves a piece of work, "for instance
the building of a house, being carried on by two kinds of workers, of which
one group was five feet four inches high, the other sixteen feet high."
("Origin of Species" (6th edition), page 232.)
Although the ant is a small animal as compared with man or with the Irish
Elk, the "soldier" with its relatively enormous jaws is hardly less heavily
burdened than the Elk with its antlers, and in the ant's case, too, a
strengthening of the skeleton, of the muscles, the nerves of the head, and
of the legs must have taken place parallel with the enlargement of the
jaws. HARMONIOUS ADAPTATION (coadaptation) has here been active in a high
degree, and yet these "soldiers" are sterile! There thus remains nothing
for it but to refer all their adaptations, positive and negative alike, to
processes of selection which have taken place in the rudiments of the
workers within the egg and sperm-cells of their parents. There is no way
out of the difficulty except the one Darwin pointed out. He himself did
not find the solution of the riddle at once. At first he believed that the
case of the workers among social insects presented "the most serious
special difficulty" in the way of his theory of natural selection; and it
was only after it had become clear to him, that it was not the sterile
insects themselves but their parents that were selected, according as they
produced more or less well adapted workers, that he was able to refer to
this very case of the conditions among ants "IN ORDER TO SHOW THE POWER OF
NATURAL SELECTION" ("Origin of Species", page 233; see also edition 1, page
242.). He explains his view by a simple but interesting illustration.
Gardeners have produced, by means of long continued artificial selection, a
variety of Stock, which bears entirely double, and therefore infertile
flowers (Ibid. page 230.). Nevertheless the variety continues to be
reproduced from seed, because in addition to the double and infertile
flowers, the seeds always produce a certain number of single, fertile
blossoms, and these are used to reproduce the double variety. These single
and fertile plants correspond "to the males and females of an ant-colony,
the infertile plants, which are regularly produced in large numbers, to the
neuter workers of the colony."
This illustration is entirely apt, the only difference between the two
cases consisting in the fact that the variation in the flower is not a
useful, but a disadvantageous one, which can only be preserved by
artificial selection on the part of the gardener, while the transformations
that have taken place parallel with the sterility of the ants are useful,
since they procure for the colony an advantage in the struggle for
existence, and they are therefore preserved by natural selection. Even the
sterility itself in this case is not disadvantageous, since the fertility
of the true females has at the same time considerably increased. We may
therefore regard the sterile forms of ants, which have gradually been
adapted in several directions to varying functions, AS A CERTAIN PROOF that
selection really takes place in the germ-cells of the fathers and mothers
of the workers, and that SPECIAL COMPLEXES OF PRIMORDIA (IDS) are present
in the workers and in the males and females, and these complexes contain
the primordia of the individual parts (DETERMINANTS). But since all living
entities vary, the determinants must also vary, now in a favourable, now in
an unfavourable direction. If a female produces eggs, which contain
favourably varying determinants in the worker-ids, then these eggs will
give rise to workers modified in the favourable direction, and if this
happens with many females, the colony concerned will contain a better kind
of worker than other colonies.
I digress here in order to give an account of the intimate processes,
which, according to my view, take place within the germ-plasm, and which I
have called "GERMINAL SELECTION." These processes are of importance since
they form the roots of variation, which in its turn is the root of natural
selection. I cannot here do more than give a brief outline of the theory
in order to show how the Darwin-Wallace theory of selection has gained
support from it.
With others, I regard the minimal amount of substance which is contained
within the nucleus of the germ-cells, in the form of rods, bands, or
granules, as the GERM-SUBSTANCE or GERM-PLASM, and I call the individual
granules IDS. There is always a multiplicity of such ids present in the
nucleus, either occurring individually, or united in the form of rods or
bands (chromosomes). Each id contains the primary constituents of a WHOLE
individual, so that several ids are concerned in the development of a new
individual.
In every being of complex structure thousands of primary constituents must
go to make up a single id; these I call DETERMINANTS, and I mean by this
name very small individual particles, far below the limits of microscopic
visibility, vital units which feed, grow, and multiply by division. These
determinants control the parts of the developing embryo,--in what manner
need not here concern us. The determinants differ among themselves, those
of a muscle are differently constituted from those of a nerve-cell or a
glandular cell, etc., and every determinant is in its turn made up of
minute vital units, which I call BIOPHORS, or the bearers of life.
According to my view, these determinants not only assimilate, like every
other living unit, but they VARY in the course of their growth, as every
living unit does; they may vary qualitatively if the elements of which they
are composed vary, they may grow and divide more or less rapidly, and their
variations give rise to CORRESPONDING variations of the organ, cell, or
cell-group which they determine. That they are undergoing ceaseless
fluctuations in regard to size and quality seems to me the inevitable
consequence of their unequal nutrition; for although the germ-cell as a
whole usually receives sufficient nutriment, minute fluctuations in the
amount carried to different parts within the germ-plasm cannot fail to
occur.
Now, if a determinant, for instance of a sensory cell, receives for a
considerable time more abundant nutriment than before, it will grow more
rapidly--become bigger, and divide more quickly, and, later, when the id
concerned develops into an embryo, this sensory cell will become stronger
than in the parents, possibly even twice as strong. This is an instance of
a HEREDITARY INDIVIDUAL VARIATION, arising from the germ.
The nutritive stream which, according to our hypothesis, favours the
determinant N by chance, that is, for reasons unknown to us, may remain
strong for a considerable time, or may decrease again; but even in the
latter case it is conceivable that the ascending movement of the
determinant may continue, because the strengthened determinant now ACTIVELY
nourishes itself more abundantly,--that is to say, it attracts the
nutriment to itself, and to a certain extent withdraws it from its fellow-
determinants. In this way, it may--as it seems to me--get into PERMANENT
UPWARD MOVEMENT, AND ATTAIN A DEGREE OF STRENGTH FROM WHICH THERE IS NO
FALLING BACK. Then positive or negative selection sets in, favouring the
variations which are advantageous, setting aside those which are
disadvantageous.
In a similar manner a DOWNWARD variation of the determinants may take
place, if its progress be started by a diminished flow of nutriment. The
determinants which are weakened by this diminished flow will have less
affinity for attracting nutriment because of their diminished strength, and
they will assimilate more feebly and grow more slowly, unless chance
streams of nutriment help them to recover themselves. But, as will
presently be shown, a change of direction cannot take place at EVERY stage
of the degenerative process. If a certain critical stage of downward
progress be passed, even favourable conditions of food-supply will no
longer suffice permanently to change the direction of the variation. Only
two cases are conceivable; if the determinant corresponds to a USEFUL
organ, only its removal can bring back the germ-plasm to its former level;
therefore personal selection removes the id in question, with its
determinants, from the germ-plasm, by causing the elimination of the
individual in the struggle for existence. But there is another conceivable
case; the determinants concerned may be those of an organ which has become
USELESS, and they will then continue unobstructed, but with exceeding
slowness, along the downward path, until the organ becomes vestigial, and
finally disappears altogether.
The fluctuations of the determinants hither and thither may thus be
transformed into a lasting ascending or descending movement; and THIS IS
THE CRUCIAL POINT OF THESE GERMINAL PROCESSES.
This is not a fantastic assumption; we can read it in the fact of the
degeneration of disused parts. USELESS ORGANS ARE THE ONLY ONES WHICH ARE
NOT HELPED TO ASCEND AGAIN BY PERSONAL SELECTION, AND THEREFORE IN THEIR
CASE ALONE CAN WE FORM ANY IDEA OF HOW THE PRIMARY CONSTITUENTS BEHAVE,
WHEN THEY ARE SUBJECT SOLELY TO INTRA-GERMINAL FORCES.
The whole determinant system of an id, as I conceive it, is in a state of
continual fluctuation upwards and downwards. In most cases the
fluctuations will counteract one another, because the passive streams of
nutriment soon change, but in many cases the limit from which a return is
possible will be passed, and then the determinants concerned will continue
to vary in the same direction, till they attain positive or negative
selection-value. At this stage personal selection intervenes and sets
aside the variation if it is disadvantageous, or favours--that is to say,
preserves--it if it is advantageous. Only THE DETERMINANT OF A USELESS
ORGAN IS UNINFLUENCED BY PERSONAL SELECTION, and, as experience shows, it
sinks downwards; that is, the organ that corresponds to it degenerates very
slowly but uninterruptedly till, after what must obviously be an immense
stretch of time, it disappears from the germ-plasm altogether.
Thus we find in the fact of the degeneration of disused parts the proof
that not all the fluctuations of a determinant return to equilibrium again,
but that, when the movement has attained to a certain strength, it
continues IN THE SAME DIRECTION. We have entire certainty in regard to
this as far as the downward progress is concerned, and we must assume it
also in regard to ascending variations, as the phenomena of artificial
selection certainly justify us in doing. If the Japanese breeders were
able to lengthen the tail feathers of the cock to six feet, it can only
have been because the determinants of the tail-feathers in the germ-plasm
had already struck out a path of ascending variation, and this movement was
taken advantage of by the breeder, who continually selected for
reproduction the individuals in which the ascending variation was most
marked. For all breeding depends upon the unconscious selection of
germinal variations.
Of course these germinal processes cannot be proved mathematically, since
we cannot actually see the play of forces of the passive fluctuations and
their causes. We cannot say how great these fluctuations are, and how
quickly or slowly, how regularly or irregularly they change. Nor do we
know how far a determinant must be strengthened by the passive flow of the
nutritive stream if it is to be beyond the danger of unfavourable
variations, or how far it must be weakened passively before it loses the
power of recovering itself by its own strength. It is no more possible to
bring forward actual proofs in this case than it was in regard to the
selection-value of the initial stages of an adaptation. But if we consider
that all heritable variations must have their roots in the germ-plasm, and
further, that when personal selection does not intervene, that is to say,
in the case of parts which have become useless, a degeneration of the part,
and therefore also of its determinant must inevitably take place; then we
must conclude that processes such as I have assumed are running their
course within the germ-plasm, and we can do this with as much certainty as
we were able to infer, from the phenomena of adaptation, the selection-
value of their initial stages. The fact of the degeneration of disused
parts seems to me to afford irrefutable proof that the fluctuations within
the germ-plasm ARE THE REAL ROOT OF ALL HEREDITARY VARIATION, and the
preliminary condition for the occurrence of the Darwin-Wallace factor of
selection. Germinal selection supplies the stones out of which personal
selection builds her temples and palaces: ADAPTATIONS. The importance for
the theory of the process of degeneration of disused parts cannot be over-
estimated, especially when it occurs in sterile animal forms, where we are
free from the doubt as to the alleged LAMARCKIAN FACTOR which is apt to
confuse our ideas in regard to other cases.
If we regard the variation of the many determinants concerned in the
transformation of the female into the sterile worker as having come about
through the gradual transformation of the ids into worker-ids, we shall see
that the germ-plasm of the sexual ants must contain three kinds of ids,
male, female, and worker ids, or if the workers have diverged into soldiers
and nest-builders, then four kinds. We understand that the worker-ids
arose because their determinants struck out a useful path of variation,
whether upward or downward, and that they continued in this path until the
highest attainable degree of utility of the parts determined was reached.
But in addition to the organs of positive or negative selection-value,
there were some which were indifferent as far as the success and especially
the functional capacity of the workers was concerned: wings, ovarian
tubes, receptaculum seminis, a number of the facets of the eye, perhaps
even the whole eye. As to the ovarian tubes it is possible that their
degeneration was an advantage for the workers, in saving energy, and if so
selection would favour the degeneration; but how could the presence of eyes
diminish the usefulness of the workers to the colony? or the minute
receptaculum seminis, or even the wings? These parts have therefore
degenerated BECAUSE THEY WERE OF NO FURTHER VALUE TO THE INSECT. But if
selection did not influence the setting aside of these parts because they
were neither of advantage nor of disadvantage to the species, then the
Darwinian factor of selection is here confronted with a puzzle which it
cannot solve alone, but which at once becomes clear when germinal selection
is added. For the determinants of organs that have no further value for
the organism, must, as we have already explained, embark on a gradual
course of retrograde development.
In ants the degeneration has gone so far that there are no wing-rudiments
present in ANY species, as is the case with so many butterflies, flies, and
locusts, but in the larvae the imaginal discs of the wings are still laid
down. With regard to the ovaries, degeneration has reached different
levels in different species of ants, as has been shown by the researches of
my former pupil, Elizabeth Bickford. In many species there are twelve
ovarian tubes, and they decrease from that number to one; indeed, in one
species no ovarian tube at all is present. So much at least is certain
from what has been said, that in this case EVERYTHING depends on the
fluctuations of the elements of the germ-plasm. Germinal selection, here
as elsewhere, presents the variations of the determinants, and personal
selection favours or rejects these, or,--if it be a question of organs
which have become useless,--it does not come into play at all, and allows
the descending variation free course.
It is obvious that even the problem of COADAPTATION IN STERILE ANIMALS can
thus be satisfactorily explained. If the determinants are oscillating
upwards and downwards in continual fluctuation, and varying more
pronouncedly now in one direction now in the other, useful variations of
every determinant will continually present themselves anew, and may, in the
course of generations, be combined with one another in various ways. But
there is one character of the determinants that greatly facilitates this
complex process of selection, that, after a certain limit has been reached,
they go on varying in the same direction. From this it follows that
development along a path once struck out may proceed without the continual
intervention of personal selection. This factor only operates, so to
speak, at the beginning, when it selects the determinants which are varying
in the right direction, and again at the end, when it is necessary to put a
check upon further variation. In addition to this, enormously long periods
have been available for all these adaptations, as the very gradual
transition stages between females and workers in many species plainly show,
and thus this process of transformation loses the marvellous and mysterious
character that seemed at the first glance to invest it, and takes rank,
without any straining, among the other processes of selection. It seems to
me that, from the facts that sterile animal forms can adapt themselves to
new vital functions, their superfluous parts degenerate, and the parts more
used adapt themselves in an ascending direction, those less used in a
descending direction, we must draw the conclusion that harmonious
adaptation here comes about WITHOUT THE COOPERATION OF THE LAMARCKIAN
PRINCIPLE. This conclusion once established, however, we have no reason to
refer the thousands of cases of harmonious adaptation, which occur in
exactly the same way among other animals or plants, to a principle, the
ACTIVE INTERVENTION OF WHICH IN THE TRANSFORMATION OF SPECIES IS NOWHERE
PROVED. WE DO NOT REQUIRE IT TO EXPLAIN THE FACTS, AND THEREFORE WE MUST
NOT ASSUME IT.
The fact of coadaptation, which was supposed to furnish the strongest
argument against the principle of selection, in reality yields the clearest
evidence in favour of it. We MUST assume it, BECAUSE NO OTHER POSSIBILITY
OF EXPLANATION IS OPEN TO US, AND BECAUSE THESE ADAPTATIONS ACTUALLY EXIST,
THAT IS TO SAY, HAVE REALLY TAKEN PLACE. With this conviction I attempted,
as far back as 1894, when the idea of germinal selection had not yet
occurred to me, to make "harmonious adaptation" (coadaptation) more easily
intelligible in some way or other, and so I was led to the idea, which was
subsequently expounded in detail by Baldwin, and Lloyd Morgan, and also by
Osborn, and Gulick as ORGANIC SELECTION. It seemed to me that it was not
necessary that all the germinal variations required for secondary
variations should have occurred SIMULTANEOUSLY, since, for instance, in the
case of the stag, the bones, muscles, sinews, and nerves would be incited
by the increasing heaviness of the antlers to greater activity in THE
INDIVIDUAL LIFE, and so would be strengthened. The antlers can only have
increased in size by very slow degrees, so that the muscles and bones may
have been able to keep pace with their growth in the individual life, until
the requisite germinal variations presented themselves. In this way a
disharmony between the increasing weight of the antlers and the parts which
support and move them would be avoided, since time would be given for the
appropriate germinal variations to occur, and so to set agoing the
HEREDITARY variation of the muscles, sinews, and bones. ("The Effect of
External Influences upon Development", Romanes Lecture, Oxford, 1894.)
I still regard this idea as correct, but I attribute less importance to
"organic selection" than I did at that time, in so far that I do not
believe that it ALONE could effect complex harmonious adaptations.
Germinal selection now seems to me to play the chief part in bringing about
such adaptations. Something the same is true of the principle I have
called "Panmixia". As I became more and more convinced, in the course of
years, that the LAMARCKIAN PRINCIPLE ought not to be called in to explain
the dwindling of disused parts, I believed that this process might be
simply explained as due to the cessation of the conservative effect of
natural selection. I said to myself that, from the moment in which a part
ceases to be of use, natural selection withdraws its hand from it, and then
it must inevitably fall from the height of its adaptiveness, because
inferior variants would have as good a chance of persisting as better ones,
since all grades of fitness of the part in question would be mingled with
one another indiscriminately. This is undoubtedly true, as Romanes pointed
out ten years before I did, and this mingling of the bad with the good
probably does bring about a deterioration of the part concerned. But it
cannot account for the steady diminution, which always occurs when a part
is in process of becoming rudimentary, and which goes on until it
ultimately disappears altogether. The process of dwindling cannot
therefore be explained as due to panmixia alone; we can only find a
sufficient explanation in germinal selection.
IV. DERIVATIVES OF THE THEORY OF SELECTION.
The impetus in all directions given by Darwin through his theory of
selection has been an immeasurable one, and its influence is still felt. It
falls within the province of the historian of science to enumerate all the
ideas which, in the last quarter of the nineteenth century, grew out of
Darwin's theories, in the endeavour to penetrate more deeply into the
problem of the evolution of the organic world. Within the narrow limits to
which this paper is restricted, I cannot attempt to discuss any of these.
V. ARGUMENTS FOR THE REALITY OF THE PROCESSES OF SELECTION.
(a) SEXUAL SELECTION.
Sexual selection goes hand in hand with natural selection. From the very
first I have regarded sexual selection as affording an extremely important
and interesting corroboration of natural selection, but, singularly enough,
it is precisely against this theory that an adverse judgment has been
pronounced in so many quarters, and it is only quite recently, and probably
in proportion as the wealth of facts in proof of it penetrates into a wider
circle, that we seem to be approaching a more general recognition of this
side of the problem of adaptation. Thus Darwin's words in his preface to
the second edition (1874) of his book, "The Descent of Man and Sexual
Selection", are being justified: "My conviction as to the operation of
natural selection remains unshaken," and further, "If naturalists were to
become more familiar with the idea of sexual selection, it would, I think,
be accepted to a much greater extent, and already it is fully and
favourably accepted by many competent judges." Darwin was able to speak
thus because he was already acquainted with an immense mass of facts,
which, taken together, yield overwhelming evidence of the validity of the
principle of sexual selection.
NATURAL SELECTION chooses out for reproduction the individuals that are
best equipped for the struggle for existence, and it does so at every stage
of development; it thus improves the species in all its stages and forms.
SEXUAL SELECTION operates only on individuals that are already capable of
reproduction, and does so only in relation to the attainment of
reproduction. It arises from the rivalry of one sex, usually the male, for
the possession of the other, usually the female. Its influence can
therefore only DIRECTLY affect one sex, in that it equips it better for
attaining possession of the other. But the effect may extend indirectly to
the female sex, and thus the whole species may be modified, without,
however, becoming any more capable of resistance in the struggle for
existence, for sexual selection only gives rise to adaptations which are
likely to give their possessor the victory over rivals in the struggle for
possession of the female, and which are therefore peculiar to the wooing
sex: the manifold "secondary sexual characters." The diversity of these
characters is so great that I cannot here attempt to give anything
approaching a complete treatment of them, but I should like to give a
sufficient number of examples to make the principle itself, in its various
modes of expression, quite clear.
One of the chief preliminary postulates of sexual selection is the unequal
number of individuals in the two sexes, for if every male immediately finds
his mate there can be no competition for the possession of the female.
Darwin has shown that, for the most part, the inequality between the sexes
is due simply to the fact that there are more males than females, and
therefore the males must take some pains to secure a mate. But the
inequality does not always depend on the numerical preponderance of the
males, it is often due to polygamy; for, if one male claims several
females, the number of females in proportion to the rest of the males will
be reduced. Since it is almost always the males that are the wooers, we
must expect to find the occurrence of secondary sexual characters chiefly
among them, and to find it especially frequent in polygamous species. And
this is actually the case.
If we were to try to guess--without knowing the facts--what means the male
animals make use of to overcome their rivals in the struggle for the
possession of the female, we might name many kinds of means, but it would
be difficult to suggest any which is not actually employed in some animal
group or other. I begin with the mere difference in strength, through
which the male of many animals is so sharply distinguished from the female,
as, for instance, the lion, walrus, "sea-elephant," and others. Among
these the males fight violently for the possession of the female, who falls
to the victor in the combat. In this simple case no one can doubt the
operation of selection, and there is just as little room for doubt as to
the selection-value of the initial stages of the variation. Differences in
bodily strength are apparent even among human beings, although in their
case the struggle for the possession of the female is no longer decided by
bodily strength alone.
Combats between male animals are often violent and obstinate, and the
employment of the natural weapons of the species in this way has led to
perfecting of these, e.g. the tusks of the boar, the antlers of the stag,
and the enormous, antler-like jaws of the stag-beetle. Here again it is
impossible to doubt that variations in these organs presented themselves,
and that these were considerable enough to be decisive in combat, and so to
lead to the improvement of the weapon.
Among many animals, however, the females at first withdraw from the males;
they are coy, and have to be sought out, and sometimes held by force. This
tracking and grasping of the females by the males has given rise to many
different characters in the latter, as, for instance, the larger eyes of
the male bee, and especially of the males of the Ephemerids (May-flies),
some species of which show, in addition to the usual compound eyes, large,
so-called turban-eyes, so that the whole head is covered with seeing
surfaces. In these species the females are very greatly in the minority (1-
100), and it is easy to understand that a keen competition for them must
take place, and that, when the insects of both sexes are floating freely in
the air, an unusually wide range of vision will carry with it a decided
advantage. Here again the actual adaptations are in accordance with the
preliminary postulates of the theory. We do not know the stages through
which the eye has passed to its present perfected state, but, since the
number of simple eyes (facets) has become very much greater in the male
than in the female, we may assume that their increase is due to a gradual
duplication of the determinants of the ommatidium in the germ-plasm, as I
have already indicated in regard to sense-organs in general. In this case,
again, the selection-value of the initial stages hardly admits of doubt;
better vision DIRECTLY secures reproduction.
In many cases THE ORGAN OF SMELL shows a similar improvement. Many lower
Crustaceans (Daphnidae) have better developed organs of smell in the male
sex. The difference is often slight and amounts only to one or two
olfactory filaments, but certain species show a difference of nearly a
hundred of these filaments (Leptodora). The same thing occurs among
insects.
We must briefly consider the clasping or grasping organs which have
developed in the males among many lower Crustaceans, but here natural
selection plays its part along with sexual selection, for the union of the
sexes is an indispensable condition for the maintenance of the species, and
as Darwin himself pointed out, in many cases the two forms of selection
merge into each other. This fact has always seemed to me to be a proof of
natural selection, for, in regard to sexual selection, it is quite obvious
that the victory of the best-equipped could have brought about the
improvement only of the organs concerned, the factors in the struggle, such
as the eye and the olfactory organ.
We come now to the EXCITANTS; that is, to the group of sexual characters
whose origin through processes of selection has been most frequently called
in question. We may cite the LOVE-CALLS produced by many male insects,
such as crickets and cicadas. These could only have arisen in animal
groups in which the female did not rapidly flee from the male, but was
inclined to accept his wooing from the first. Thus, notes like the
chirping of the male cricket serve to entice the females. At first they
were merely the signal which showed the presence of a male in the
neighbourhood, and the female was gradually enticed nearer and nearer by
the continued chirping. The male that could make himself heard to the
greatest distance would obtain the largest following, and would transmit
the beginnings, and, later, the improvement of his voice to the greatest
number of descendants. But sexual excitement in the female became
associated with the hearing of the love-call, and then the sound-producing
organ of the male began to improve, until it attained to the emission of
the long-drawn-out soft notes of the mole-cricket or the maenad-like cry of
the cicadas. I cannot here follow the process of development in detail,
but will call attention to the fact that the original purpose of the voice,
the announcing of the male's presence, became subsidiary, and the exciting
of the female became the chief goal to be aimed at. The loudest singers
awakened the strongest excitement, and the improvement resulted as a matter
of course. I conceive of the origin of bird-song in a somewhat similar
manner, first as a means of enticing, then of exciting the female.
One more kind of secondary sexual character must here be mentioned: the
odour which emanates from so many animals at the breeding season. It is
possible that this odour also served at first merely to give notice of the
presence of individuals of the other sex, but it soon became an excitant,
and as the individuals which caused the greatest degree of excitement were
preferred, it reached as high a pitch of perfection as was possible to it.
I shall confine myself here to the comparatively recently discovered
fragrance of butterflies. Since Fritz Muller found out that certain
Brazilian butterflies gave off fragrance "like a flower," we have become
acquainted with many such cases, and we now know that in all lands, not
only many diurnal Lepidoptera but nocturnal ones also give off a delicate
odour, which is agreeable even to man. The ethereal oil to which this
fragrance is due is secreted by the skin-cells, usually of the wing, as I
showed soon after the discovery of the SCENT-SCALES. This is the case in
the males; the females have no SPECIAL scent-scales recognisable as such by
their form, but they must, nevertheless, give off an extremely delicate
fragrance, although our imperfect organ of smell cannot perceive it, for
the males become aware of the presence of a female, even at night, from a
long distance off, and gather round her. We may therefore conclude, that
both sexes have long given forth a very delicate perfume, which announced
their presence to others of the same species, and that in many species (NOT
IN ALL) these small beginnings became, in the males, particularly strong
scent-scales of characteristic form (lute, brush, or lyre-shaped). At
first these scales were scattered over the surface of the wing, but
gradually they concentrated themselves, and formed broad, velvety bands, or
strong, prominent brushes, and they attained their highest pitch of
evolution when they became enclosed within pits or folds of the skin, which
could be opened to let the delicious fragrance stream forth suddenly
towards the female. Thus in this case also we see that characters, the
original use of which was to bring the sexes together, and so to maintain
the species, have been evolved in the males into means for exciting the
female. And we can hardly doubt, that the females are most readily enticed
to yield to the butterfly that sends out the strongest fragrance,--that is
to say, that excites them to the highest degree. It is a pity that our
organs of smell are not fine enough to examine the fragrance of male
Lepidoptera in general, and to compare it with other perfumes which attract
these insects. (See Poulton, "Essays on Evolution", 1908, pages 316, 317.)
As far as we can perceive them they resemble the fragrance of flowers, but
there are Lepidoptera whose scent suggests musk. A smell of musk is also
given off by several plants: it is a sexual excitant in the musk-deer, the
musk-sheep, and the crocodile.
As far as we know, then, it is perfumes similar to those of flowers that
the male Lepidoptera give off in order to entice their mates, and this is a
further indication that animals, like plants, can to a large extent meet
the claims made upon them by life, and produce the adaptations which are
most purposive,--a further proof, too, of my proposition that the useful
variations, so to speak, are ALWAYS THERE. The flowers developed the
perfumes which entice their visitors, and the male Lepidoptera developed
the perfumes which entice and excite their mates.
There are many pretty little problems to be solved in this connection, for
there are insects, such as some flies, that are attracted by smells which
are unpleasant to us, like those from decaying flesh and carrion. But
there are also certain flowers, some orchids for instance, which give forth
no very agreeable odour, but one which is to us repulsive and disgusting;
and we should therefore expect that the males of such insects would give
off a smell unpleasant to us, but there is no case known to me in which
this has been demonstrated.
In cases such as we have discussed, it is obvious that there is no possible
explanation except through selection. This brings us to the last kind of
secondary sexual characters, and the one in regard to which doubt has been
most frequently expressed,--decorative colours and decorative forms, the
brilliant plumage of the male pheasant, the humming-birds, and the bird of
Paradise, as well as the bright colours of many species of butterfly, from
the beautiful blue of our little Lycaenidae to the magnificent azure of the
large Morphinae of Brazil. In a great many cases, though not by any means
in all, the male butterflies are "more beautiful" than the females, and in
the Tropics in particular they shine and glow in the most superb colours.
I really see no reason why we should doubt the power of sexual selection,
and I myself stand wholly on Darwin's side. Even though we certainly
cannot assume that the females exercise a conscious choice of the
"handsomest" mate, and deliberate like the judges in a court of justice
over the perfections of their wooers, we have no reason to doubt that
distinctive forms (decorative feathers) and colours have a particularly
exciting effect upon the female, just as certain odours have among animals
of so many different groups, including the butterflies. The doubts which
existed for a considerable time, as a result of fallacious experiments, as
to whether the colours of flowers really had any influence in attracting
butterflies have now been set at rest through a series of more careful
investigations; we now know that the colours of flowers are there on
account of the butterflies, as Sprengel first showed, and that the blossoms
of Phanerogams are selected in relation to them, as Darwin pointed out.
Certainly it is not possible to bring forward any convincing proof of the
origin of decorative colours through sexual selection, but there are many
weighty arguments in favour of it, and these form a body of presumptive
evidence so strong that it almost amounts to certainty.
In the first place, there is the analogy with other secondary sexual
characters. If the song of birds and the chirping of the cricket have been
evolved through sexual selection, if the penetrating odours of male
animals,--the crocodile, the musk-deer, the beaver, the carnivores, and,
finally, the flower-like fragrances of the butterflies have been evolved to
their present pitch in this way, why should decorative colours have arisen
in some other way? Why should the eye be less sensitive to SPECIFICALLY
MALE colours and other VISIBLE signs ENTICING TO THE FEMALE, than the
olfactory sense to specifically male odours, or the sense of hearing to
specifically male sounds? Moreover, the decorative feathers of birds are
almost always spread out and displayed before the female during courtship.
I have elsewhere ("The Evolution Theory", London, 1904, I. page 219.)
pointed out that decorative colouring and sweet-scentedness may replace one
another in Lepidoptera as well as in flowers, for just as some modestly
coloured flowers (mignonette and violet) have often a strong perfume, while
strikingly coloured ones are sometimes quite devoid of fragrance, so we
find that the most beautiful and gaily-coloured of our native Lepidoptera,
the species of Vanessa, have no scent-scales, while these are often
markedly developed in grey nocturnal Lepidoptera. Both attractions may,
however, be combined in butterflies, just as in flowers. Of course, we
cannot explain why both means of attraction should exist in one genus, and
only one of them in another, since we do not know the minutest details of
the conditions of life of the genera concerned. But from the sporadic
distribution of scent-scales in Lepidoptera, and from their occurrence or
absence in nearly related species, we may conclude that fragrance is a
relatively MODERN acquirement, more recent than brilliant colouring.
One thing in particular that stamps decorative colouring as a product of
selection is ITS GRADUAL INTENSIFICATION by the addition of new spots,
which we can quite well observe, because in many cases the colours have
been first acquired by the males, and later transmitted to the females by
inheritance. The scent-scales are never thus transmitted, probably for the
same reason that the decorative colours of many birds are often not
transmitted to the females: because with these they would be exposed to
too great elimination by enemies. Wallace was the first to point out that
in species with concealed nests the beautiful feathers of the male occurred
in the female also, as in the parrots, for instance, but this is not the
case in species which brood on an exposed nest. In the parrots one can
often observe that the general brilliant colouring of the male is found in
the female, but that certain spots of colour are absent, and these have
probably been acquired comparatively recently by the male and have not yet
been transmitted to the female.
Isolation of the group of individuals which is in process of varying is
undoubtedly of great value in sexual selection, for even a solitary
conspicuous variation will become dominant much sooner in a small isolated
colony, than among a large number of members of a species.
Anyone who agrees with me in deriving variations from germinal selection
will regard that process as an essential aid towards explaining the
selection of distinctive courtship-characters, such as coloured spots,
decorative feathers, horny outgrowths in birds and reptiles, combs,
feather-tufts, and the like, since the beginnings of these would be
presented with relative frequency in the struggle between the determinants
within the germ-plasm. The process of transmission of decorative feathers
to the female results, as Darwin pointed out and illustrated by interesting
examples, in the COLOUR-TRANSFORMATION OF A WHOLE SPECIES, and this
process, as the phyletically older colouring of young birds shows, must, in
the course of thousands of years, have repeated itself several times in a
line of descent.
If we survey the wealth of phenomena presented to us by secondary sexual
characters, we can hardly fail to be convinced of the truth of the
principle of sexual selection. And certainly no one who has accepted
natural selection should reject sexual selection, for, not only do the two
processes rest upon the same basis, but they merge into one another, so
that it is often impossible to say how much of a particular character
depends on one and how much on the other form of selection.
(b) NATURAL SELECTION.
An actual proof of the theory of sexual selection is out of the question,
if only because we cannot tell when a variation attains to selection-value.
It is certain that a delicate sense of smell is of value to the male moth
in his search for the female, but whether the possession of one additional
olfactory hair, or of ten, or of twenty additional hairs leads to the
success of its possessor we are unable to tell. And we are groping even
more in the dark when we discuss the excitement caused in the female by
agreeable perfumes, or by striking and beautiful colours. That these do
make an impression is beyond doubt; but we can only assume that slight
intensifications of them give any advantage, and we MUST assume this SINCE
OTHERWISE SECONDARY SEXUAL CHARACTERS REMAIN INEXPLICABLE.
The same thing is true in regard to natural selection. It is not possible
to bring forward any actual proof of the selection-value of the initial
stages, and the stages in the increase of variations, as has been already
shown. But the selection-value of a finished adaptation can in many cases
be statistically determined. Cesnola and Poulton have made valuable
experiments in this direction. The former attached forty-five individuals
of the green, and sixty-five of the brown variety of the praying mantis
(Mantis religiosa), by a silk thread to plants, and watched them for
seventeen days. The insects which were on a surface of a colour similar to
their own remained uneaten, while twenty-five green insects on brown parts
of plants had all disappeared in eleven days.
The experiments of Poulton and Sanders ("Report of the British Association"
(Bristol, 1898), London, 1899, pages 906-909.) were made with 600 pupae of
Vanessa urticae, the "tortoise-shell butterfly." The pupae were
artificially attached to nettles, tree-trunks, fences, walls, and to the
ground, some at Oxford, some at St Helens in the Isle of Wight. In the
course of a month 93 per cent of the pupae at Oxford were killed, chiefly
by small birds, while at St Helens 68 per cent perished. The experiments
showed very clearly that the colour and character of the surface on which
the pupa rests--and thus its own conspicuousness--are of the greatest
importance. At Oxford only the four pupae which were fastened to nettles
emerged; all the rest--on bark, stones and the like--perished. At St
Helens the elimination was as follows: on fences where the pupae were
conspicuous, 92 per cent; on bark, 66 per cent; on walls, 54 per cent; and
among nettles, 57 per cent. These interesting experiments confirm our
views as to protective coloration, and show further, THAT THE RATIO OF
ELIMINATION IN THE SPECIES IS A VERY HIGH ONE, AND THAT THEREFORE SELECTION
MUST BE VERY KEEN.
We may say that the process of selection follows as a logical necessity
from the fulfilment of the three preliminary postulates of the theory:
variability, heredity, and the struggle for existence, with its enormous
ratio of elimination in all species. To this we must add a fourth factor,
the INTENSIFICATION of variations which Darwin established as a fact, and
which we are now able to account for theoretically on the basis of germinal
selection. It may be objected that there is considerable uncertainty about
this LOGICAL proof, because of our inability to demonstrate the selection-
value of the initial stages and the individual stages of increase. We have
therefore to fall back on PRESUMPTIVE EVIDENCE. This is to be found in THE
INTERPRETATIVE VALUE OF THE THEORY. Let us consider this point in greater
detail.
In the first place, it is necessary to emphasise what is often overlooked,
namely, that the theory not only explains the TRANSFORMATIONS of species,
it also explains THEIR REMAINING THE SAME; in addition to the principle of
varying, it contains within itself that of PERSISTING. It is part of the
essence of selection, that it not only causes a part to VARY till it has
reached its highest pitch of adaptation, but that it MAINTAINS IT AT THIS
PITCH. THIS CONSERVING INFLUENCE OF NATURAL SELECTION is of great
importance, and was early recognised by Darwin; it follows naturally from
the principle of the survival of the fittest.
We understand from this how it is that a species which has become fully
adapted to certain conditions of life ceases to vary, but remains
"constant," as long as the conditions of life FOR IT remain unchanged,
whether this be for thousands of years, or for whole geological epochs.
But the most convincing proof of the power of the principle of selection
lies in the innumerable multitude of phenomena which cannot be explained in
any other way. To this category belong all structures which are only
PASSIVELY of advantage to the organism, because none of these can have
arisen by the alleged LAMARCKIAN PRINCIPLE. These have been so often
discussed that we need do no more than indicate them here. Until quite
recently the sympathetic coloration of animals--for instance, the whiteness
of Arctic animals--was referred, at least in part, to the DIRECT influence
of external factors, but the facts can best be explained by referring them
to the processes of selection, for then it is unnecessary to make the
gratuitous assumption that many species are sensitive to the stimulus of
cold and that others are not. The great majority of Arctic land-animals,
mammals and birds, are white, and this proves that they were all able to
present the variation which was most useful for them. The sable is brown,
but it lives in trees, where the brown colouring protects and conceals it
more effectively. The musk-sheep (Ovibos moschatus) is also brown, and
contrasts sharply with the ice and snow, but it is protected from beasts of
prey by its gregarious habit, and therefore it is of advantage to be
visible from as great a distance as possible. That so many species have
been able to give rise to white varieties does not depend on a special
sensitiveness of the skin to the influence of cold, but to the fact that
Mammals and Birds have a general tendency to vary towards white. Even with
us, many birds--starlings, blackbirds, swallows, etc.--occasionally produce
white individuals, but the white variety does not persist, because it
readily falls a victim to the carnivores. This is true of white fawns,
foxes, deer, etc. The whiteness, therefore, arises from internal causes,
and only persists when it is useful. A great many animals living in a
GREEN ENVIRONMENT have become clothed in green, especially insects,
caterpillars, and Mantidae, both persecuted and persecutors.
That it is not the direct effect of the environment which calls forth the
green colour is shown by the many kinds of caterpillar which rest on leaves
and feed on them, but are nevertheless brown. These feed by night and
betake themselves through the day to the trunk of the tree, and hide in the
furrows of the bark. We cannot, however, conclude from this that they were
UNABLE to vary towards green, for there are Arctic animals which are white
only in winter and brown in summer (Alpine hare, and the ptarmigan of the
Alps), and there are also green leaf-insects which remain green only while
they are young and difficult to see on the leaf, but which become brown
again in the last stage of larval life, when they have outgrown the leaf.
They then conceal themselves by day, sometimes only among withered leaves
on the ground, sometimes in the earth itself. It is interesting that in
one genus, Chaerocampa, one species is brown in the last stage of larval
life, another becomes brown earlier, and in many species the last stage is
not wholly brown, a part remaining green. Whether this is a case of a
double adaptation, or whether the green is being gradually crowded out by
the brown, the fact remains that the same species, even the same
individual, can exhibit both variations. The case is the same with many of
the leaf-like Orthoptera, as, for instance, the praying mantis (Mantis
religiosa) which we have already mentioned.
But the best proofs are furnished by those often-cited cases in which the
insect bears a deceptive resemblance to another object. We now know many
such cases, such as the numerous imitations of green or withered leaves,
which are brought about in the most diverse ways, sometimes by mere
variations in the form of the insect and in its colour, sometimes by an
elaborate marking, like that which occurs in the Indian leaf-butterflies,
Kallima inachis. In the single butterfly-genus Anaea, in the woods of
South America, there are about a hundred species which are all gaily
coloured on the upper surface, and on the reverse side exhibit the most
delicate imitation of the colouring and pattern of a leaf, generally
without any indication of the leaf-ribs, but extremely deceptive
nevertheless. Anyone who has seen only one such butterfly may doubt
whether many of the insignificant details of the marking can really be of
advantage to the insect. Such details are for instance the apparent holes
and splits in the apparently dry or half-rotten leaf, which are usually due
to the fact that the scales are absent on a circular or oval patch so that
the colourless wing-membrane lies bare, and one can look through the spot
as through a window. Whether the bird which is seeking or pursuing the
butterflies takes these holes for dewdrops, or for the work of a devouring
insect, does not affect the question; the mirror-like spot undoubtedly
increases the general deceptiveness, for the same thing occurs in many
leaf-butterflies, though not in all, and in some cases it is replaced in
quite a peculiar manner. In one species of Anaea (A. divina), the resting
butterfly looks exactly like a leaf out of the outer edge of which a large
semicircular piece has been eaten, possibly by a caterpillar; but if we
look more closely it is obvious that there is no part of the wing absent,
and that the semicircular piece is of a clear, pale yellow colour, while
the rest of the wing is of a strongly contrasted dark brown.
But the deceptive resemblance may be caused in quite a different manner. I
have often speculated as to what advantage the brilliant white C could give
to the otherwise dusky-coloured "Comma butterfly" (Grapta C. album).
Poulton's recent observations ("Proc. Ent. Soc"., London, May 6, 1903.)
have shown that this represents the imitation of a crack such as is often
seen in dry leaves, and is very conspicuous because the light shines
through it.
The utility obviously lies in presenting to the bird the very familiar
picture of a broken leaf with a clear shining slit, and we may conclude,
from the imitation of such small details, that the birds are very sharp
observers and that the smallest deviation from the usual arrests their
attention and incites them to closer investigation. It is obvious that
such detailed--we might almost say such subtle--deceptive resemblances
could only have come about in the course of long ages through the
acquirement from time to time of something new which heightened the already
existing resemblance.
In face of facts like these there can be no question of chance, and no one
has succeeded so far in finding any other explanation to replace that by
selection. For the rest, the apparent leaves are by no means perfect
copies of a leaf; many of them only represent the torn or broken piece, or
the half or two-thirds of a leaf, but then the leaves themselves frequently
do not present themselves to the eye as a whole, but partially concealed
among other leaves. Even those butterflies which, like the species of
Kallima and Anaea, represent the whole of a leaf with stalk, ribs, apex,
and the whole breadth, are not actual copies which would satisfy a
botanist; there is often much wanting. In Kallima the lateral ribs of the
leaf are never all included in the markings; there are only two or three on
the left side and at most four or five on the right, and in many
individuals these are rather obscure, while in others they are
comparatively distinct. This furnishes us with fresh evidence in favour of
their origin through processes of selection, for a botanically perfect
picture could not arise in this way; there could only be a fixing of such
details as heightened the deceptive resemblance.
Our postulate of origin through selection also enables us to understand why
the leaf-imitation is on the lower surface of the wing in the diurnal
Lepidoptera, and on the upper surface in the nocturnal forms, corresponding
to the attitude of the wings in the resting position of the two groups.
The strongest of all proofs of the theory, however, is afforded by cases of
true "mimicry," those adaptations discovered by Bates in 1861, consisting
in the imitation of one species by another, which becomes more and more
like its model. The model is always a species that enjoys some special
protection from enemies, whether because it is unpleasant to taste, or
because it is in some way dangerous.
It is chiefly among insects and especially among butterflies that we find
the greatest number of such cases. Several of these have been minutely
studied, and every detail has been investigated, so that it is difficult to
understand how there can still be disbelief in regard to them. If the many
and exact observations which have been carefully collected and critically
discussed, for instance by Poulton ("Essays on Evolution", 1889-1907,
Oxford, 1908, passim, e.g. page 269.) were thoroughly studied, the
arguments which are still frequently urged against mimicry would be found
untenable; we can hardly hope to find more convincing proof of the
actuality of the processes of selection than these cases put into our
hands. The preliminary postulates of the theory of mimicry have been
disputed, for instance, that diurnal butterflies are persecuted and eaten
by birds, but observations specially directed towards this point in India,
Africa, America and Europe have placed it beyond all doubt. If it were
necessary I could myself furnish an account of my own observations on this
point.
In the same way it has been established by experiment and observation in
the field that in all the great regions of distribution there are
butterflies which are rejected by birds and lizards, their chief enemies,
on account of their unpleasant smell or taste. These butterflies are
usually gaily and conspicuously coloured and thus--as Wallace first
interpreted it--are furnished with an easily recognisable sign: a sign of
unpalatableness or WARNING COLOURS. If they were not thus recognisable
easily and from a distance, they would frequently be pecked at by birds,
and then rejected because of their unpleasant taste; but as it is, the
insect-eaters recognise them at once as unpalatable booty and ignore them.
Such IMMUNE (The expression does not refer to all the enemies of this
butterfly; against ichneumon-flies, for instance, their unpleasant smell
usually gives no protection.) species, wherever they occur, are imitated by
other palatable species, which thus acquire a certain degree of protection.
It is true that this explanation of the bright, conspicuous colours is only
a hypothesis, but its foundations,--unpalatableness, and the liability of
other butterflies to be eaten,--are certain, and its consequences--the
existence of mimetic palatable forms--confirm it in the most convincing
manner. Of the many cases now known I select one, which is especially
remarkable, and which has been thoroughly investigated, Papilio dardanus
(merope), a large, beautiful, diurnal butterfly which ranges from Abyssinia
throughout the whole of Africa to the south coast of Cape Colony.
The males of this form are everywhere ALMOST the same in colour and in form
of wings, save for a few variations in the sparse black markings on the
pale yellow ground. But the females occur in several quite different forms
and colourings, and one of these only, the Abyssinian form, is like the
male, while the other three or four are MIMETIC, that is to say, they copy
a butterfly of quite a different family the Danaids, which are among the
IMMUNE forms. In each region the females have thus copied two or three
different immune species. There is much that is interesting to be said in
regard to these species, but it would be out of keeping with the general
tenor of this paper to give details of this very complicated case of
polymorphism in P. dardanus. Anyone who is interested in the matter will
find a full and exact statement of the case in as far as we know it, in
Poulton's "Essays on Evolution" (pages 373-375). (Professor Poulton has
corrected some wrong descriptions which I had unfortunately overlooked in
the Plates of my book "Vortrage uber Descendenztheorie", and which refer to
Papilio dardanus (merope). These mistakes are of no importance as far as
and understanding of the mimicry-theory is concerned, but I hope shortly to
be able to correct them in a later edition.) I need only add that three
different mimetic female forms have been reared from the eggs of a single
female in South Africa. The resemblance of these forms to their immune
models goes so far that even the details of the LOCAL forms of the models
are copied by the mimetic species.
It remains to be said that in Madagascar a butterfly, Papilio meriones,
occurs, of which both sexes are very similar in form and markings to the
non-mimetic male of P. dardanus, so that it probably represents the
ancestor of this latter species.
In face of such facts as these every attempt at another explanation must
fail. Similarly all the other details of the case fulfil the preliminary
postulates of selection, and leave no room for any other interpretation.
That the males do not take on the protective colouring is easily explained,
because they are in general more numerous, and the females are more
important for the preservation of the species, and must also live longer in
order to deposit their eggs. We find the same state of things in many
other species, and in one case (Elymnias undularis) in which the male is
also mimetically coloured, it copies quite a differently coloured immune
species from the model followed by the female. This is quite intelligible
when we consider that if there were TOO MANY false immune types, the birds
would soon discover that there were palatable individuals among those with
unpalatable warning colours. Hence the imitation of different immune
species by Papilio dardanus!
I regret that lack of space prevents my bringing forward more examples of
mimicry and discussing them fully. But from the case of Papilio dardanus
alone there is much to be learnt which is of the highest importance for our
understanding of transformations. It shows us chiefly what I once called,
somewhat strongly perhaps, THE OMNIPOTENCE OF NATURAL SELECTION in answer
to an opponent who had spoken of its "inadequacy." We here see that one
and the same species is capable of producing four or five different
patterns of colouring and marking; thus the colouring and marking are not,
as has often been supposed, a necessary outcome of the specific nature of
the species, but a true adaptation, which cannot arise as a direct effect
of climatic conditions, but solely through what I may call the sorting out
of the variations produced by the species, according to their utility.
That caterpillars may be either green or brown is already something more
than could have been expected according to the old conception of species,
but that one and the same butterfly should be now pale yellow, with black;
now red with black and pure white; now deep black with large, pure white
spots; and again black with a large ochreous-yellow spot, and many small
white and yellow spots; that in one sub-species it may be tailed like the
ancestral form, and in another tailless like its Danaid model,--all this
shows a far-reaching capacity for variation and adaptation that wide never
have expected if we did not see the facts before us. How it is possible
that the primary colour-variations should thus be intensified and combined
remains a puzzle even now; we are reminded of the modern three-colour
printing,--perhaps similar combinations of the primary colours take place
in this case; in any case the direction of these primary variations is
determined by the artist whom we know as natural selection, for there is no
other conceivable way in which the model could affect the butterfly that is
becoming more and more like it. The same climate surrounds all four forms
of female; they are subject to the same conditions of nutrition. Moreover,
Papilio dardanus is by no means the only species of butterfly which
exhibits different kinds of colour-pattern on its wings. Many species of
the Asiatic genus Elymnias have on the upper surface a very good imitation
of an immune Euploeine (Danainae), often with a steel-blue ground-colour,
while the under surface is well concealed when the butterfly is at rest,--
thus there are two kinds of protective coloration each with a different
meaning! The same thing may be observed in many non-mimetic butterflies,
for instance in all our species of Vanessa, in which the under side shows a
grey-brown or brownish-black protective coloration, but we do not yet know
with certainty what may be the biological significance of the gaily
coloured upper surface.
In general it may be said that mimetic butterflies are comparatively rare
species, but there are exceptions, for instance Limenitis archippus in
North America, of which the immune model (Danaida plexippus) also occurs in
enormous numbers.
In another mimicry-category the imitators are often more numerous than the
models, namely in the case of the imitation of DANGEROUS INSECTS by
harmless species. Bees and wasps are dreaded for their sting, and they are
copied by harmless flies of the genera Eristalis and Syrphus, and these
mimics often occur in swarms about flowering plants without damage to
themselves or to their models; they are feared and are therefore left
unmolested.
In regard also to the FAITHFULNESS OF THE COPY the facts are quite in
harmony with the theory, according to which the resemblance must have
arisen and increased BY DEGREES. We can recognise this in many cases, for
even now the mimetic species show very VARYING DEGREES OF RESEMBLANCE to
their immune model. If we compare, for instance, the many different
imitators of Danaida chrysippus we find that, with their brownish-yellow
ground-colour, and the position and size, and more or less sharp limitation
of their clear marginal spots, they have reached very different degrees of
nearness to their model. Or compare the female of Elymnias undularis with
its model Danaida genutia; there is a general resemblance, but the marking
of the Danaida is very roughly imitated in Elymnias.
Another fact that bears out the theory of mimicry is, that even when the
resemblance in colour-pattern is very great, the WING-VENATION, which is
so constant, and so important in determining the systematic position of
butterflies, is never affected by the variation. The pursuers of the
butterfly have no time to trouble about entomological intricacies.
I must not pass over a discovery of Poulton's which is of great theoretical
importance--that mimetic butterflies may reach the same effect by very
different means. ("Journ. Linn. Soc. London (Zool.)", Vol. XXVI. 1898,
pages 598-602.) Thus the glass-like transparency of the wing of a certain
Ithomiine (Methona) and its Pierine mimic (Dismorphia orise) depends on a
diminution in the size of the scales; in the Danaine genus Ituna it is due
to the fewness of the scales, and in a third imitator, a moth (Castnia
linus var. heliconoides) the glass-like appearance of the wing is due
neither to diminution nor to absence of scales, but to their absolute
colourlessness and transparency, and to the fact that they stand upright.
In another moth mimic (Anthomyza) the arrangement of the transparent scales
is normal. Thus it is not some unknown external influence that has brought
about the transparency of the wing in these five forms, as has sometimes
been supposed. Nor is it a hypothetical INTERNAL evolutionary tendency,
for all three vary in a different manner. The cause of this agreement can
only lie in selection, which preserves and intensifies in each species the
favourable variations that present themselves. The great faithfulness of
the copy is astonishing in these cases, for it is not THE WHOLE wing which
is transparent; certain markings are black in colour, and these contrast
sharply with the glass-like ground. It is obvious that the pursuers of
these butterflies must be very sharp-sighted, for otherwise the agreement
between the species could never have been pushed so far. The less the
enemies see and observe, the more defective must the imitation be, and if
they had been blind, no visible resemblance between the species which
required protection could ever have arisen.
A seemingly irreconcilable contradiction to the mimicry theory is presented
in the following cases, which were known to Bates, who, however, never
succeeded in bringing them into line with the principle of mimicry.
In South America there are, as we have already said, many mimics of the
immune Ithomiinae (or as Bates called them Heliconidae). Among these there
occur not merely species which are edible, and thus require the protection
of a disguise, but others which are rejected on account of their
unpalatableness. How could the Ithomiine dress have developed in their
case, and of what use is it, since the species would in any case be immune?
In Eastern Brazil, for instance, there are four butterflies, which bear a
most confusing resemblance to one another in colour, marking, and form of
wing, and all four are unpalatable to birds. They belong to four different
genera and three sub-families, and we have to inquire: Whence came this
resemblance and what end does it serve? For a long time no satisfactory
answer could be found, but Fritz Muller (In "Kosmos", 1879, page 100.),
seventeen years after Bates, offered a solution to the riddle, when he
pointed out that young birds could not have an instinctive knowledge of the
unpalatableness of the Ithomiines, but must learn by experience which
species were edible and which inedible. Thus each young bird must have
tasted at least one individual of each inedible species and discovered its
unpalatability, before it learnt to avoid, and thus to spare the species.
But if the four species resemble each other very closely the bird will
regard them all as of the same kind, and avoid them all. Thus there
developed a process of selection which resulted in the survival of the
Ithomiine-like individuals, and in so great an increase of resemblance
between the four species, that they are difficult to distinguish one from
another even in a collection. The advantage for the four species, living
side by side as they do e.g. in Bahia, lies in the fact that only one
individual from the MIMICRY-RING ("inedible association") need be tasted by
a young bird, instead of at least four individuals, as would otherwise be
the case. As the number of young birds is great, this makes a considerable
difference in the ratio of elimination.
These interesting mimicry-rings (trusts), which have much significance for
the theory, have been the subject of numerous and careful investigations,
and at least their essential features are now fully established. Muller
took for granted, without making any investigations, that young birds only
learn by experience to distinguish between different kinds of victims. But
Lloyd Morgan's ("Habit and Instinct", London, 1896.) experiments with young
birds proved that this is really the case, and at the same time furnished
an additional argument against the LAMARCKIAN PRINCIPLE.
In addition to the mimicry-rings first observed in South America, others
have been described from Tropical India by Moore, and by Poulton and Dixey
from Africa, and we may expect to learn many more interesting facts in this
connection. Here again the preliminary postulates of the theory are
satisfied. And how much more that would lead to the same conclusion might
be added!
As in the case of mimicry many species have come to resemble one another
through processes of selection, so we know whole classes of phenomena in
which plants and animals have become adapted to one another, and have thus
been modified to a considerable degree. I refer particularly to the
relation between flowers and insects; but as there is an article on "The
Biology of Flowers" in this volume, I need not discuss the subject, but
will confine myself to pointing out the significance of these remarkable
cases for the theory of selection. Darwin has shown that the originally
inconspicuous blossoms of the phanerogams were transformed into flowers
through the visits of insects, and that, conversely, several large orders
of insects have been gradually modified by their association with flowers,
especially as regards the parts of their body actively concerned. Bees and
butterflies in particular have become what they are through their relation
to flowers. In this case again all that is apparently contradictory to the
theory can, on closer investigation, be beautifully interpreted in
corroboration of it. Selection can give rise only to what is of use to the
organism actually concerned, never to what is of use to some other
organism, and we must therefore expect to find that in flowers only
characters of use to THEMSELVES have arisen, never characters which are of
use to insects only, and conversely that in the insects characters useful
to them and not merely to the plants would have originated. For a long
time it seemed as if an exception to this rule existed in the case of the
fertilisation of the yucca blossoms by a little moth, Pronuba yuccasella.
This little moth has a sickle-shaped appendage to its mouth-parts which
occurs in no other Lepidopteron, and which is used for pushing the yellow
pollen into the opening of the pistil, thus fertilising the flower. Thus
it appears as if a new structure, which is useful only to the plant, has
arisen in the insect. But the difficulty is solved as soon as we learn
that the moth lays its eggs in the fruit-buds of the Yucca, and that the
larvae, when they emerge, feed on the developing seeds. In effecting the
fertilisation of the flower the moth is at the same time making provision
for its own offspring, since it is only after fertilisation that the seeds
begin to develop. There is thus nothing to prevent our referring this
structural adaptation in Pronuba yuccasella to processes of selection,
which have gradually transformed the maxillary palps of the female into the
sickle-shaped instrument for collecting the pollen, and which have at the
same time developed in the insect the instinct to press the pollen into the
pistil.
In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any other
explanation, which, now that the facts are so well known, could be regarded
as a serious rival to it. That selection is a factor, and a very powerful
factor in the evolution of organisms, can no longer be doubted. Even
although we cannot bring forward formal proofs of it IN DETAIL, cannot
calculate definitely the size of the variations which present themselves,
and their selection-value, cannot, in short, reduce the whole process to a
mathematical formula, yet we must assume selection, because it is the only
possible explanation applicable to whole classes of phenomena, and because,
on the other hand, it is made up of factors which we know can be proved
actually to exist, and which, IF they exist, must of logical necessity
cooperate in the manner required by the theory. WE MUST ACCEPT IT BECAUSE
THE PHENOMENA OF EVOLUTION AND ADAPTATION MUST HAVE A NATURAL BASIS, AND
BECAUSE IT IS THE ONLY POSSIBLE EXPLANATION OF THEM. (This has been
discussed in many of my earlier works. See for instance "The All-
Sufficiency of Natural Selection, a reply to Herbert Spencer", London,
1893.)
Many people are willing to admit that selection explains adaptations, but
they maintain that only a part of the phenomena are thus explained, because
everything does not depend upon adaptation. They regard adaptation as, so
to speak, a special effort on the part of Nature, which she keeps in
readiness to meet particularly difficult claims of the external world on
organisms. But if we look at the matter more carefully we shall find that
adaptations are by no means exceptional, but that they are present
everywhere in such enormous numbers, that it would be difficult in regard
to any structure whatever, to prove that adaptation had NOT played a part
in its evolution.
How often has the senseless objection been urged against selection that it
can create nothing, it can only reject. It is true that it cannot create
either the living substance or the variations of it; both must be given.
But in rejecting one thing it preserves another, intensifies it, combines
it, and in this way CREATES what is new. EVERYTHING in organisms depends
on adaptation; that is to say, everything must be admitted through the
narrow door of selection, otherwise it can take no part in the building up
of the whole. But, it is asked, what of the direct effect of external
conditions, temperature, nutrition, climate and the like? Undoubtedly
these can give rise to variations, but they too must pass through the door
of selection, and if they cannot do this they are rejected, eliminated from
the constitution of the species.
It may, perhaps, be objected that such external influences are often of a
compelling power, and that every animal MUST submit to them, and that thus
selection has no choice and can neither select nor reject. There may be
such cases; let us assume for instance that the effect of the cold of the
Arctic regions was to make all the mammals become black; the result would
be that they would all be eliminated by selection, and that no mammals
would be able to live there at all. But in most cases a certain percentage
of animals resists these strong influences, and thus selection secures a
foothold on which to work, eliminating the unfavourable variation, and
establishing a useful colouring, consistent with what is required for the
maintenance of the species.
Everything depends upon adaptation! We have spoken much of adaptation in
colouring, in connection with the examples brought into prominence by
Darwin, because these are conspicuous, easily verified, and at the same
time convincing for the theory of selection. But is it only desert and
polar animals whose colouring is determined through adaptation? Or the
leaf-butterflies, and the mimetic species, or the terrifying markings, and
"warning-colours" and a thousand other kinds of sympathetic colouring? It
is, indeed, never the colouring alone which makes up the adaptation; the
structure of the animal plays a part, often a very essential part, in the
protective disguise, and thus MANY variations may cooperate towards ONE
common end. And it is to be noted that it is by no means only external
parts that are changed; internal parts are ALWAYS modified at the same
time--for instance, the delicate elements of the nervous system on which
depend the INSTINCT of the insect to hold its wings, when at rest, in a
perfectly definite position, which, in the leaf-butterfly, has the effect
of bringing the two pieces on which the marking occurs on the anterior and
posterior wing into the same direction, and thus displaying as a whole the
fine curve of the midrib on the seeming leaf. But the wing-holding instinct
is not regulated in the same way in all leaf-butterflies; even our
indigenous species of Vanessa, with their protective ground-colouring, have
quite a distinctive way of holding their wings so that the greater part of
the anterior wing is covered by the posterior when the butterfly is at
rest. But the protective colouring appears on the posterior wing and on
the tip of the anterior, TO PRECISELY THE DISTANCE TO WHICH IT IS LEFT
UNCOVERED. This occurs, as Standfuss has shown, in different degree in our
two most nearly allied species, the uncovered portion being smaller in V.
urticae than in V. polychloros. In this case, as in most leaf-butterflies,
the holding of the wing was probably the primary character; only after that
was thoroughly established did the protective marking develop. In any
case, the instinctive manner of holding the wings is associated with the
protective colouring, and must remain as it is if the latter is to be
effective. How greatly instincts may change, that is to say, may be
adapted, is shown by the case of the Noctuid "shark" moth, Xylina vetusta.
This form bears a most deceptive resemblance to a piece of rotten wood, and
the appearance is greatly increased by the modification of the innate
impulse to flight common to so many animals, which has here been
transformed into an almost contrary instinct. This moth does not fly away
from danger, but "feigns death," that is, it draws antennae, legs and wings
close to the body, and remains perfectly motionless. It may be touched,
picked up, and thrown down again, and still it does not move. This
remarkable instinct must surely have developed simultaneously with the
wood-colouring; at all events, both cooperating variations are now present,
and prove that both the external and the most minute internal structure
have undergone a process of adaptation.
The case is the same with all structural variations of animal parts, which
are not absolutely insignificant. When the insects acquired wings they
must also have acquired the mechanism with which to move them--the
musculature, and the nervous apparatus necessary for its automatic
regulation. All instincts depend upon compound reflex mechanisms and are
just as indispensable as the parts they have to set in motion, and all may
have arisen through processes of selection if the reasons which I have
elsewhere given for this view are correct. ("The Evolution Theory",
London, 1904, page 144.)
Thus there is no lack of adaptations within the organism, and particularly
in its most important and complicated parts, so that we may say that there
is no actively functional organ that has not undergone a process of
adaptation relative to its function and the requirements of the organism.
Not only is every gland structurally adapted, down to the very minutest
histological details, to its function, but the function is equally minutely
adapted to the needs of the body. Every cell in the mucous lining of the
intestine is exactly regulated in its relation to the different nutritive
substances, and behaves in quite a different way towards the fats, and
towards nitrogenous substances, or peptones.
I have elsewhere called attention to the many adaptations of the whale to
the surrounding medium, and have pointed out--what has long been known, but
is not universally admitted, even now--that in it a great number of
important organs have been transformed in adaptation to the peculiar
conditions of aquatic life, although the ancestors of the whale must have
lived, like other hair-covered mammals, on land. I cited a number of these
transformations--the fish-like form of the body, the hairlessness of the
skin, the transformation of the fore-limbs to fins, the disappearance of
the hind-limbs and the development of a tail fin, the layer of blubber
under the skin, which affords the protection from cold necessary to a warm-
blooded animal, the disappearance of the ear-muscles and the auditory
passages, the displacement of the external nares to the forehead for the
greater security of the breathing-hole during the brief appearance at the
surface, and certain remarkable changes in the respiratory and circulatory
organs which enable the animal to remain for a long time under water. I
might have added many more, for the list of adaptations in the whale to
aquatic life is by no means exhausted; they are found in the histological
structure and in the minutest combinations in the nervous system. For it
is obvious that a tail-fin must be used in quite a different way from a
tail, which serves as a fly-brush in hoofed animals, or as an aid to
springing in the kangaroo or as a climbing organ; it will require quite
different reflex-mechanisms and nerve-combinations in the motor centres.
I used this example in order to show how unnecessary it is to assume a
special internal evolutionary power for the phylogenesis of species, for
this whole order of whales is, so to speak, MADE UP OF ADAPTATIONS; it
deviates in many essential respects from the usual mammalian type, and all
the deviations are adaptations to aquatic life. But if precisely the most
essential features of the organisation thus depend upon adaptation, what is
left for a phyletic force to do, since it is these essential features of
the structure it would have to determine? There are few people now who
believe in a phyletic evolutionary power, which is not made up of the
forces known to us--adaptation and heredity--but the conviction that EVERY
part of an organism depends upon adaptation has not yet gained a firm
footing. Nevertheless, I must continue to regard this conception as the
correct one, as I have long done.
I may be permitted one more example. The feather of a bird is a marvellous
structure, and no one will deny that as a whole it depends upon adaptation.
But what part of it DOES NOT depend upon adaptation? The hollow quill, the
shaft with its hard, thin, light cortex, and the spongy substance within
it, its square section compared with the round section of the quill, the
flat barbs, their short, hooked barbules which, in the flight-feathers,
hook into one another with just sufficient firmness to resist the pressure
of the air at each wing-beat, the lightness and firmness of the whole
apparatus, the elasticity of the vane, and so on. And yet all this belongs
to an organ which is only passively functional, and therefore can have
nothing to do with the LAMARCKIAN PRINCIPLE. Nor can the feather have
arisen through some magical effect of temperature, moisture, electricity,
or specific nutrition, and thus selection is again our only anchor of
safety.
But--it will be objected--the substance of which the feather consists, this
peculiar kind of horny substance, did not first arise through selection in
the course of the evolution of the birds, for it formed the covering of the
scales of their reptilian ancestors. It is quite true that a similar
substance covered the scales of the Reptiles, but why should it not have
arisen among them through selection? Or in what other way could it have
arisen, since scales are also passively useful parts? It is true that if
we are only to call adaptation what has been acquired by the species we
happen to be considering, there would remain a great deal that could not be
referred to selection; but we are postulating an evolution which has
stretched back through aeons, and in the course of which innumerable
adaptations took place, which had not merely ephemeral persistence in a
genus, a family or a class, but which was continued into whole Phyla of
animals, with continual fresh adaptations to the special conditions of each
species, family, or class, yet with persistence of the fundamental
elements. Thus the feather, once acquired, persisted in all birds, and the
vertebral column, once gained by adaptation in the lowest forms, has
persisted in all the Vertebrates, from Amphioxus upwards, although with
constant readaptation to the conditions of each particular group. Thus
everything we can see in animals is adaptation, whether of to-day, or of
yesterday, or of ages long gone by; every kind of cell, whether glandular,
muscular, nervous, epidermic, or skeletal, is adapted to absolutely
definite and specific functions, and every organ which is composed of these
different kinds of cells contains them in the proper proportions, and in
the particular arrangement which best serves the function of the organ; it
is thus adapted to its function.
All parts of the organism are tuned to one another, that is, THEY ARE
ADAPTED TO ONE ANOTHER, and in the same way THE ORGANISM AS A WHOLE IS
ADAPTED TO THE CONDITIONS OF ITS LIFE, AND IT IS SO AT EVERY STAGE OF ITS
EVOLUTION.
But all adaptations CAN be referred to selection; the only point that
remains doubtful is whether they all MUST be referred to it.
However that may be, whether the LAMARCKIAN PRINCIPLE is a factor that has
cooperated with selection in evolution, or whether it is altogether
fallacious, the fact remains, that selection is the cause of a great part
of the phyletic evolution of organisms on our earth. Those who agree with
me in rejecting the LAMARCKIAN PRINCIPLE will regard selection as the only
GUIDING factor in evolution, which creates what is new out of the
transmissible variations, by ordering and arranging these, selecting them
in relation to their number and size, as the architect does his building-
stones so that a particular style must result. ("Variation under
Domestication", 1875 II. pages 426, 427.) But the building-stones
themselves, the variations, have their basis in the influences which cause
variation in those vital units which are handed on from one generation to
another, whether, taken together they form the WHOLE organism, as in
Bacteria and other low forms of life, or only a germ-substance, as in
unicellular and multicellular organisms. (The Author and Editor are
indebted to Professor Poulton for kindly assisting in the revision of the
proof of this Essay.)
IV. VARIATION.
By HUGO DE VRIES,
Professor of Botany in the University of Amsterdam.
I. DIFFERENT KINDS OF VARIABILITY.
Before Darwin, little was known concerning the phenomena of variability.
The fact, that hardly two leaves on a tree were exactly the same, could not
escape observation: small deviations of the same kind were met with
everywhere, among individuals as well as among the organs of the same
plant. Larger aberrations, spoken of as monstrosities, were for a long
time regarded as lying outside the range of ordinary phenomena. A special
branch of inquiry, that of Teratology, was devoted to them, but it
constituted a science by itself, sometimes connected with morphology, but
having scarcely any bearing on the processes of evolution and heredity.
Darwin was the first to take a broad survey of the whole range of
variations in the animal and vegetable kingdoms. His theory of Natural
Selection is based on the fact of variability. In order that this
foundation should be as strong as possible he collected all the facts,
scattered in the literature of his time, and tried to arrange them in a
scientific way. He succeeded in showing that variations may be grouped
along a line of almost continuous gradations, beginning with simple
differences in size and ending with monstrosities. He was struck by the
fact that, as a rule, the smaller the deviations, the more frequently they
appear, very abrupt breaks in characters being of rare occurrence.
Among these numerous degrees of variability Darwin was always on the look
out for those which might, with the greatest probability, be considered as
affording material for natural selection to act upon in the development of
new species. Neither of the extremes complied with his conceptions. He
often pointed out, that there are a good many small fluctuations, which in
this respect must be absolutely useless. On the other hand, he strongly
combated the belief, that great changes would be necessary to explain the
origin of species. Some authors had propounded the idea that highly
adapted organs, e.g. the wings of a bird, could not have been developed in
any other way than by a comparatively sudden modification of a well defined
and important kind. Such a conception would allow of great breaks or
discontinuity in the evolution of highly differentiated animals and plants,
shortening the time for the evolution of the whole organic kingdom and
getting over numerous difficulties inherent in the theory of slow and
gradual progress. It would, moreover, account for the genetic relation of
the larger groups of both animals and plants. It would, in a word,
undoubtedly afford an easy means of simplifying the problem of descent with
modification.
Darwin, however, considered such hypotheses as hardly belonging to the
domain of science; they belong, he said, to the realm of miracles. That
species have a capacity for change is admitted by all evolutionists; but
there is no need to invoke modifications other than those represented by
ordinary variability. It is well known that in artificial selection this
tendency to vary has given rise to numerous distinct races, and there is no
reason for denying that it can do the same in nature, by the aid of natural
selection. On both lines an advance may be expected with equal
probability.
His main argument, however, is that the most striking and most highly
adapted modifications may be acquired by successive variations. Each of
these may be slight, and they may affect different organs, gradually
adapting them to the same purpose. The direction of the adaptations will
be determined by the needs in the struggle for life, and natural selection
will simply exclude all such changes as occur on opposite or deviating
lines. In this way, it is not variability itself which is called upon to
explain beautiful adaptations, but it is quite sufficient to suppose that
natural selection has operated during long periods in the same way.
Eventually, all the acquired characters, being transmitted together, would
appear to us, as if they had all been simultaneously developed.
Correlations must play a large part in such special evolutions: when one
part is modified, so will be other parts. The distribution of nourishment
will come in as one of the causes, the reactions of different organs to the
same external influences as another. But no doubt the more effective cause
is that of the internal correlations, which, however, are still but dimly
understood. Darwin repeatedly laid great stress on this view, although a
definite proof of its correctness could not be given in his time. Such
proof requires the direct observation of a mutation, and it should be
stated here that even the first observations made in this direction have
clearly confirmed Darwin's ideas. The new evening primroses which have
sprung in my garden from the old form of Oenothera Lamarckiana, and which
have evidently been derived from it, in each case, by a single mutation, do
not differ from their parent species in one character only, but in almost
all their organs and qualities. Oenothera gigas, for example, has stouter
stems and denser foliage; the leaves are larger and broader; its thick
flower-buds produce gigantic flowers, but only small fruits with large
seeds. Correlative changes of this kind are seen in all my new forms, and
they lend support to the view that in the gradual development of highly
adapted structures, analogous correlations may have played a large part.
They easily explain large deviations from an original type, without
requiring the assumption of too many steps.
Monstrosities, as their name implies, are widely different in character
from natural species; they cannot, therefore, be adduced as evidence in the
investigation of the origin of species. There is no doubt that they may
have much in common as regards their manner of origin, and that the origin
of species, once understood, may lead to a better understanding of the
monstrosities. But the reverse is not true, at least not as regards the
main lines of development. Here, it is clear, monstrosities cannot have
played a part of any significance.
Reversions, or atavistic changes, would seem to give a better support to
the theory of descent through modifications. These have been of paramount
importance on many lines of evolution of the animal as well as of the
vegetable kingdom. It is often assumed that monocotyledons are descended
from some lower group of dicotyledons, probably allied to that which
includes the buttercup family. On this view the monocotyledons must be
assumed to have lost the cambium and all its influence on secondary growth,
the differentiation of the flower into calyx and corolla, the second
cotyledon or seed-leaf and several other characters. Losses of characters
such as these may have been the result of abrupt changes, but this does not
prove that the characters themselves have been produced with equal
suddenness. On the contrary, Darwin shows very convincingly that a
modification may well be developed by a series of steps, and afterwards
suddenly disappear. Many monstrosities, such as those represented by
twisted stems, furnish direct proofs in support of this view, since they
are produced by the loss of one character and this loss implies secondary
changes in a large number of other organs and qualities.
Darwin criticises in detail the hypothesis of great and abrupt changes and
comes to the conclusion that it does not give even a shadow of an
explanation of the origin of species. It is as improbable as it is
unnecessary.
Sports and spontaneous variations must now be considered. It is well known
that they have produced a large number of fine horticultural varieties.
The cut-leaved maple and many other trees and shrubs with split leaves are
known to have been produced at a single step; this is true in the case of
the single-leaf strawberry plant and of the laciniate variety of the
greater celandine: many white flowers, white or yellow berries and
numerous other forms had a similar origin. But changes such as these do
not come under the head of adaptations, as they consist for the most part
in the loss of some quality or organ belonging to the species from which
they were derived. Darwin thinks it impossible to attribute to this cause
the innumerable structures, which are so well adapted to the habits of life
of each species. At the present time we should say that such adaptations
require progressive modifications, which are additions to the stock of
qualities already possessed by the ancestors, and cannot, therefore, be
explained on the ground of a supposed analogy with sports, which are for
the most part of a retrogressive nature.
Excluding all these more or less sudden changes, there remains a long
series of gradations of variability, but all of these are not assumed by
Darwin to be equally fit for the production of new species. In the first
place, he disregards all mere temporary variations, such as size, albinism,
etc.; further, he points out that very many species have almost certainly
been produced by steps, not greater, and probably not very much smaller,
than those separating closely related varieties. For varieties are only
small species. Next comes the question of polymorphic species: their
occurrence seems to have been a source of much doubt and difficulty in
Darwin's mind, although at present it forms one of the main supports of the
prevailing explanation of the origin of new species. Darwin simply states
that this kind of variability seems to be of a peculiar nature; since
polymorphic species are now in a stable condition their occurrence gives no
clue as to the mode of origin of new species. Polymorphic species are the
expression of the result of previous variability acting on a large scale;
but they now simply consist of more or less numerous elementary species,
which, as far as we know, do not at present exhibit a larger degree of
variability than any other more uniform species. The vernal whitlow-grass
(Draba verna) and the wild pansy are the best known examples; both have
spread over almost the whole of Europe and are split up into hundreds of
elementary forms. These sub-species show no signs of any extraordinary
degree of variability, when cultivated under conditions necessary for the
exclusion of inter-crossing. Hooker has shown, in the case of some ferns
distributed over still wider areas, that the extinction of some of the
intermediate forms in such groups would suffice to justify the elevation of
the remaining types to the rank of distinct species. Polymorphic species
may now be regarded as the link which unites ordinary variability with the
historical production of species. But it does not appear that they had
this significance for Darwin; and, in fact, they exhibit no phenomena which
could explain the processes by which one species has been derived from
another. By thus narrowing the limits of the species-producing variability
Darwin was led to regard small deviations as the source from which natural
selection derives material upon which to act. But even these are not all
of the same type, and Darwin was well aware of the fact.
It should here be pointed out that in order to be selected, a change must
first have been produced. This proposition, which now seems self-evident,
has, however, been a source of much difference of opinion among Darwin's
followers. The opinion that natural selection produces changes in useful
directions has prevailed for a long time. In other words, it was assumed
that natural selection, by the simple means of singling out, could induce
small and useful changes to increase and to reach any desired degree of
deviation from the original type. In my opinion this view was never
actually held by Darwin. It is in contradiction with the acknowledged aim
of all his work,--the explanation of the origin of species by means of
natural forces and phenomena only. Natural selection acts as a sieve; it
does not single out the best variations, but it simply destroys the larger
number of those which are, from some cause or another, unfit for their
present environment. In this way it keeps the strains up to the required
standard, and, in special circumstances, may even improve them.
Returning to the variations which afford the material for the sieving-
action of natural selection, we may distinguish two main kinds. It is true
that the distinction between these was not clear at the time of Darwin, and
that he was unable to draw a sharp line between them. Nevertheless, in
many cases, he was able to separate them, and he often discussed the
question which of the two would be the real source of the differentiation
of species. Certain variations constantly occur, especially such as are
connected with size, weight, colour, etc. They are usually too small for
natural selection to act upon, having hardly any influence in the struggle
for life: others are more rare, occurring only from time to time, perhaps
once or twice in a century, perhaps even only once in a thousand years.
Moreover, these are of another type, not simply affecting size, number or
weight, but bringing about something new, which may be useful or not.
Whenever the variation is useful natural selection will take hold of it and
preserve it; in other cases the variation may either persist or disappear.
In his criticism of miscellaneous objections brought forward against the
theory of natural selection after the publication of the first edition of
"The Origin of Species", Darwin stated his view on this point very
clearly:--"The doctrine of natural selection or the survival of the
fittest, which implies that when variations or individual differences of a
beneficial nature happen to arise, these will be preserved." ("Origin of
Species" (6th edition), page 169, 1882.) In this sentence the words
"HAPPEN TO ARISE" appear to me of prominent significance. They are
evidently due to the same general conception which prevailed in Darwin's
Pangenesis hypothesis. (Cf. de Vries, "Intracellulare Pangenesis", page
73, Jena, 1889, and "Die Mutationstheorie", I. page 63. Leipzig, 1901.)
A distinction is indicated between ordinary fluctuations which are always
present, and such variations as "happen to arise" from time to time. ((I
think it right to point out that the interpretation of this passage from
the "Origin" by Professor de Vries is not accepted as correct either by Mr
Francis Darwin or by myself. We do not believe that Darwin intended to
draw any distinction between TWO TYPES of variation; the words "when
variations or individual differences of a beneficial nature happen to
arise" are not in our opinion meant to imply a distinction between ordinary
fluctuations and variations which "happen to arise," but we believe that
"or" is here used in the sense of ALIAS. With the permission of Professor
de Vries, the following extract is quoted from a letter in which he replied
to the objection raised to his reading of the passage in question:
"As to your remarks on the passage on page 6, I agree that it is now
impossible to see clearly how far Darwin went in his distinction of the
different kinds of variability. Distinctions were only dimly guessed at by
him. But in our endeavour to arrive at a true conception of his view I
think that the chapter on Pangenesis should be our leading guide, and that
we should try to interpret the more difficult passages by that chapter. A
careful and often repeated study of the Pangenesis hypothesis has convinced
me that Darwin, when he wrote that chapter, was well aware that ordinary
variability has nothing to do with evolution, but that other kinds of
variation were necessary. In some chapters he comes nearer to a clear
distinction than in others. To my mind the expression 'happen to arise' is
the sharpest indication of his inclining in this direction. I am quite
convinced that numerous expressions in his book become much clearer when
looked at in this way."
The statement in this passage that "Darwin was well aware that ordinary
variability has nothing to do with evolution, but that other kinds of
variation were necessary" is contradicted by many passages in the "Origin".
A.C.S.)) The latter afford the material for natural selection to act upon
on the broad lines of organic development, but the first do not.
Fortuitous variations are the species-producing kind, which the theory
requires; continuous fluctuations constitute, in this respect, a useless
type.
Of late, the study of variability has returned to the recognition of this
distinction. Darwin's variations, which from time to time happen to arise,
are MUTATIONS, the opposite type being commonly designed fluctuations. A
large mass of facts, collected during the last few decades, has confirmed
this view, which in Darwin's time could only be expressed with much
reserve, and everyone knows that Darwin was always very careful in
statements of this kind.
From the same chapter I may here cite the following paragraph: "Thus as I
am inclined to believe, morphological differences,...such as the
arrangement of the leaves, the divisions of the flower or of the ovarium,
the position of the ovules, etc.--first appeared in many cases as
fluctuating variations, which sooner or later became constant through the
nature of the organism and of the surrounding conditions...but NOT THROUGH
NATURAL SELECTION (The italics are mine (H. de V.).); for as these
morphological characters do not affect the welfare of the species, any
slight deviation in them could not have been governed or accumulated
through this latter agency." ("Origin of Species" (6th edition), page
176.) We thus see that in Darwin's opinion, all small variations had not
the same importance. In favourable circumstances some could become
constant, but others could not.
Since the appearance of the first edition of "The Origin of Species"
fluctuating variability has been thoroughly studied by Quetelet. He
discovered the law, which governs all phenomena of organic life falling
under this head. It is a very simple law, and states that individual
variations follow the laws of probability. He proved it, in the first
place, for the size of the human body, using the measurements published for
Belgian recruits; he then extended it to various other measurements of
parts of the body, and finally concluded that it must be of universal
validity for all organic beings. It must hold true for all characters in
man, physical as well as intellectual and moral qualities; it must hold
true for the plant kingdom as well as for the animal kingdom; in short, it
must include the whole living world.
Quetelet's law may be most easily studied in those cases where the
variability relates to measure, number and weight, and a vast number of
facts have since confirmed its exactness and its validity for all kinds of
organisms, organs and qualities. But if we examine it more closely, we
find that it includes just those minute variations, which, as Darwin
repeatedly pointed out, have often no significance for the origin of
species. In the phenomena, described by Quetelet's law nothing "happens to
arise"; all is governed by the common law, which states that small
deviations from the mean type are frequent, but that larger aberrations are
rare, the rarer as they are larger. Any degree of variation will be found
to occur, if only the number of individuals studied is large enough: it is
even possible to calculate before hand, how many specimens must be compared
in order to find a previously fixed degree of deviation.
The variations, which from time to time happen to appear, are evidently not
governed by this law. They cannot, as yet, be produced at will: no
sowings of thousands or even of millions of plants will induce them,
although by such means the chance of their occurring will obviously be
increased. But they are known to occur, and to occur suddenly and
abruptly. They have been observed especially in horticulture, where they
are ranged in the large and ill-defined group called sports. Korschinsky
has collected all the evidence which horticultural literature affords on
this point. (S. Korschinsky, "Heterogenesis und Evolution", "Flora", Vol.
LXXXIX. pages 240-363, 1901.) Several cases of the first appearance of a
horticultural novelty have been recorded: this has always happened in the
same way; it appeared suddenly and unexpectedly without any definite
relation to previously existing variability. Dwarf types are one of the
commonest and most favourite varieties of flowering plants; they are not
originated by a repeated selection of the smallest specimens, but appear at
once, without intermediates and without any previous indication. In many
instances they are only about half the height of the original type, thus
constituting obvious novelties. So it is in other cases described by
Korschinsky: these sports or mutations are now recognised to be the main
source of varieties of horticultural plants.
As already stated, I do not pretend that the production of horticultural
novelties is the prototype of the origin of new species in nature. I
assume that they are, as a rule, derived from the parent species by the
loss of some organ or quality, whereas the main lines of the evolution of
the animal and vegetable kingdom are of course determined by progressive
changes. Darwin himself has often pointed out this difference. But the
saltatory origin of horticultural novelties is as yet the simplest parallel
for natural mutations, since it relates to forms and phenomena, best known
to the general student of evolution.
The point which I wish to insist upon is this. The difference between
small and ever present fluctuations and rare and more sudden variations was
clear to Darwin, although the facts known at his time were too meagre to
enable a sharp line to be drawn between these two great classes of
variability. Since Darwin's time evidence, which proves the correctness of
his view, has accumulated with increasing rapidity. Fluctuations
constitute one type; they are never absent and follow the law of chance,
but they do not afford the material from which to build new species.
Mutations, on the other hand, only happen to occur from time to time. They
do not necessarily produce greater changes than fluctuations, but such as
may become, or rather are from their very nature, constant. It is this
constancy which is the mark of specific characters, and on this basis every
new specific character may be assumed to have arisen by mutation.
Some authors have tried to show that the theory of mutation is opposed to
Darwin's views. But this is erroneous. On the contrary, it is in fullest
harmony with the great principle laid down by Darwin. In order to be acted
upon by that complex of environmental forces, which Darwin has called
natural selection, the changes must obviously first be there. The manner
in which they are produced is of secondary importance and has hardly any
bearing on the theory of descent with modification. ("Life and Letters"
II. 125.)
A critical survey of all the facts of variability of plants in nature as
well as under cultivation has led me to the conviction, that Darwin was
right in stating that those rare beneficial variations, which from time to
time happen to arise,--the now so-called mutations--are the real source of
progress in the whole realm of the organic world.
II. EXTERNAL AND INTERNAL CAUSES OF VARIABILITY.
All phenomena of animal and plant life are governed by two sets of causes;
one of these is external, the other internal. As a rule the internal
causes determine the nature of a phenomenon--what an organism can do and
what it cannot do. The external causes, on the other hand, decide when a
certain variation will occur, and to what extent its features may be
developed.
As a very clear and wholly typical instance I cite the cocks-combs
(Celosia). This race is distinguished from allied forms by its faculty of
producing the well-known broad and much twisted combs. Every single
individual possesses this power, but all individuals do not exhibit it in
its most complete form. In some cases this faculty may not be exhibited at
the top of the main stem, although developed in lateral branches: in
others it begins too late for full development. Much depends upon
nourishment and cultivation, but almost always the horticulturist has to
single out the best individuals and to reject those which do not come up to
the standard.
The internal causes are of a historical nature. The external ones may be
defined as nourishment and environment. In some cases nutrition is the
main factor, as, for instance, in fluctuating variability, but in natural
selection environment usually plays the larger part.
The internal or historical causes are constant during the life-time of a
species, using the term species in its most limited sense, as designating
the so-called elementary species or the units out of which the ordinary
species are built up. These historical causes are simply the specific
characters, since in the origin of a species one or more of these must have
been changed, thus producing the characters of the new type. These changes
must, of course, also be due partly to internal and partly to external
causes.
In contrast to these changes of the internal causes, the ordinary
variability which is exhibited during the life-time of a species is called
fluctuating variability. The name mutations or mutating variability is
then given to the changes in the specific characters. It is desirable to
consider these two main divisions of variability separately.
In the case of fluctuations the internal causes, as well as the external
ones, are often apparent. The specific characters may be designated as the
mean about which the observed forms vary. Almost every character may be
developed to a greater or a less degree, but the variations of the single
characters producing a small deviation from the mean are usually the
commonest. The limits of these fluctuations may be called wide or narrow,
according to the way we look at them, but in numerous cases the extreme on
the favoured side hardly surpasses double the value of that on the other
side. The degree of this development, for every individual and for every
organ, is dependent mainly on nutrition. Better nourishment or an
increased supply of food produces a higher development; only it is not
always easy to determine which direction is the fuller and which is the
poorer one. The differences among individuals grown from different seeds
are described as examples of individual variability, but those which may be
observed on the same plant, or on cuttings, bulbs or roots derived from one
individual are referred to as cases of partial variability. Partial
variability, therefore, determines the differences among the flowers,
fruits, leaves or branches of one individual: in the main, it follows the
same laws as individual variability, but the position of a branch on a
plant also determines its strength, and the part it may take in the
nourishment of the whole. Composite flowers and umbels therefore have, as
a rule, fewer rays on weak branches than on the strong main ones. The
number of carpels in the fruits of poppies becomes very small on the weak
lateral branches, which are produced towards the autumn, as well as on
crowded, and therefore on weakened individuals. Double flowers follow the
same rule, and numerous other instances could easily be adduced.
Mutating variability occurs along three main lines. Either a character may
disappear, or, as we now say, become latent; or a latent character may
reappear, reproducing thereby a character which was once prominent in more
or less remote ancestors. The third and most interesting case is that of
the production of quite new characters which never existed in the
ancestors. Upon this progressive mutability the main development of the
animal and vegetable kingdom evidently depends. In contrast to this, the
two other cases are called retrogressive and degressive mutability. In
nature retrogressive mutability plays a large part; in agriculture and in
horticulture it gives rise to numerous varieties, which have in the past
been preserved, either on account of their usefulness or beauty, or simply
as fancy-types. In fact the possession of numbers of varieties may be
considered as the main character of domesticated animals and cultivated
plants.
In the case of retrogressive and degressive mutability the internal cause
is at once apparent, for it is this which causes the disappearance or
reappearance of some character. With progressive mutations the case is not
so simple, since the new character must first be produced and then
displayed. These two processes are theoretically different, but they may
occur together or after long intervals. The production of the new
character I call premutation, and the displaying mutation. Both of course
must have their external as well as their internal causes, as I have
repeatedly pointed out in my work on the Mutation Theory. ("Die
Mutationstheorie", 2 vols., Leipzig, 1901.)
It is probable that nutrition plays as important a part among the external
causes of mutability as it does among those of fluctuating variability.
Observations in support of this view, however, are too scanty to allow of a
definite judgment. Darwin assumed an accumulative influence of external
causes in the case of the production of new varieties or species. The
accumulation might be limited to the life-time of a single individual, or
embrace that of two or more generations. In the end a degree of
instability in the equilibrium of one or more characters might be attained,
great enough for a character to give way under a small shock produced by
changed conditions of life. The character would then be thrown over from
the old state of equilibrium into a new one.
Characters which happen to be in this state of unstable equilibrium are
called mutable. They may be either latent or active, being in the former
case derived from old active ones or produced as new ones (by the process,
designated premutation). They may be inherited in this mutable condition
during a long series of generations. I have shown that in the case of the
evening primrose of Lamarck this state of mutability must have existed for
at least half a century, for this species was introduced from Texas into
England about the year 1860, and since then all the strains derived from
its first distribution over the several countries of Europe show the same
phenomena in producing new forms. The production of the dwarf evening
primrose, or Oenothera nanella, is assumed to be due to one of the factors,
which determines the tall stature of the parent form, becoming latent; this
would, therefore, afford an example of retrogressive mutation. Most of the
other types of my new mutants, on the other hand, seem to be due to
progressive mutability.
The external causes of this curious period of mutability are as yet wholly
unknown and can hardly be guessed at, since the origin of the Oenothera
Lamarckiana is veiled in mystery. The seeds, introduced into England about
1860, were said to have come from Texas, but whether from wild or from
cultivated plants we do not know. Nor has the species been recorded as
having been observed in the wild condition. This, however, is nothing
peculiar. The European types of Oenothera biennis and O. muricata are in
the same condition. The first is said to have been introduced from
Virginia, and the second from Canada, but both probably from plants
cultivated in the gardens of these countries. Whether the same elementary
species are still growing on those spots is unknown, mainly because the
different sub-species of the species mentioned have not been systematically
studied and distinguished.
The origin of new species, which is in part the effect of mutability, is,
however, due mainly to natural selection. Mutability provides the new
characters and new elementary species. Natural selection, on the other
hand, decides what is to live and what to die. Mutability seems to be
free, and not restricted to previously determined lines. Selection,
however, may take place along the same main lines in the course of long
geological epochs, thus directing the development of large branches of the
animal and vegetable kingdom. In natural selection it is evident that
nutrition and environment are the main factors. But it is probable that,
while nutrition may be one of the main causes of mutability, environment
may play the chief part in the decisions ascribed to natural selection.
Relations to neighbouring plants and to injurious or useful animals, have
been considered the most important determining factors ever since the time
when Darwin pointed out their prevailing influence.
From this discussion of the main causes of variability we may derive the
proposition that the study of every phenomenon in the field of heredity, of
variability, and of the origin of new species will have to be considered
from two standpoints; on one hand we have the internal causes, on the other
the external ones. Sometimes the first are more easily detected, in other
cases the latter are more accessible to investigation. But the complete
elucidation of any phenomenon of life must always combine the study of the
influence of internal with that of external causes.
III. POLYMORPHIC VARIABILITY IN CEREALS.
One of the propositions of Darwin's theory of the struggle for life
maintains that the largest amount of life can be supported on any area, by
great diversification or divergence in the structure and constitution of
its inhabitants. Every meadow and every forest affords a proof of this
thesis. The numerical proportion of the different species of the flora is
always changing according to external influences. Thus, in a given meadow,
some species will flower abundantly in one year and then almost disappear,
until, after a series of years, circumstances allow them again to multiply
rapidly. Other species, which have taken their places, will then become
rare. It follows from this principle, that notwithstanding the constantly
changing conditions, a suitable selection from the constituents of a meadow
will ensure a continued high production. But, although the principle is
quite clear, artificial selection has, as yet, done very little towards
reaching a really high standard.
The same holds good for cereals. In ordinary circumstances a field will
give a greater yield, if the crop grown consists of a number of
sufficiently differing types. Hence it happens that almost all older
varieties of wheat are mixtures of more or less diverging forms. In the
same variety the numerical composition will vary from year to year, and in
oats this may, in bad years, go so far as to destroy more than half of the
harvest, the wind-oats (Avena fatua), which scatter their grain to the
winds as soon as it ripens, increasing so rapidly that they assume the
dominant place. A severe winter, a cold spring and other extreme
conditions of life will destroy one form more completely than another, and
it is evident that great changes in the numerical composition of the
mixture may thus be brought about.
This mixed condition of the common varieties of cereals was well known to
Darwin. For him it constituted one of the many types of variability. It
is of that peculiar nature to which, in describing other groups, he applies
the term polymorphy. It does not imply that the single constituents of the
varieties are at present really changing their characters. On the other
hand, it does not exclude the possibility of such changes. It simply
states that observation shows the existence of different forms; how these
have originated is a question which it does not deal with. In his well-
known discussion of the variability of cereals, Darwin is mainly concerned
with the question, whether under cultivation they have undergone great
changes or only small ones. The decision ultimately depends on the
question, how many forms have originally been taken into cultivation.
Assuming five or six initial species, the variability must be assumed to
have been very large, but on the assumption that there were between ten and
fifteen types, the necessary range of variability is obviously much
smaller. But in regard to this point, we are of course entirely without
historical data.
Few of the varieties of wheat show conspicuous differences, although their
number is great. If we compare the differentiating characters of the
smaller types of cereals with those of ordinary wild species, even within
the same genus or family, they are obviously much less marked. All these
small characters, however, are strictly inherited, and this fact makes it
very probable that the less obvious constituents of the mixtures in
ordinary fields must be constant and pure as long as they do not
intercross. Natural crossing is in most cereals a phenomenon of rare
occurrence, common enough to admit of the production of all possible hybrid
combinations, but requiring the lapse of a long series of years to reach
its full effect.
Darwin laid great stress on this high amount of variability in the plants
of the same variety, and illustrated it by the experience of Colonel Le
Couteur ("On the Varieties, Properties, and Classification of Wheat",
Jersey, 1837.) on his farm on the isle of Jersey, who cultivated upwards of
150 varieties of wheat, which he claimed were as pure as those of any other
agriculturalist. But Professor La Gasca of Madrid, who visited him, drew
attention to aberrant ears, and pointed out, that some of them might be
better yielders than the majority of plants in the crop, whilst others
might be poor types. Thence he concluded that the isolation of the better
ones might be a means of increasing his crops. Le Couteur seems to have
considered the constancy of such smaller types after isolation as
absolutely probable, since he did not even discuss the possibility of their
being variable or of their yielding a changeable or mixed progeny. This
curious fact proves that he considered the types, discovered in his fields
by La Gasca to be of the same kind as his other varieties, which until that
time he had relied upon as being pure and uniform. Thus we see, that for
him, the variability of cereals was what we now call polymorphy. He looked
through his fields for useful aberrations, and collected twenty-three new
types of wheat. He was, moreover, clear about one point, which, on being
rediscovered after half a century, has become the starting-point for the
new Swedish principle of selecting agricultural plants. It was the
principle of single-ear sowing, instead of mixing the grains of all the
selected ears together. By sowing each ear on a separate plot he intended
not only to multiply them, but also to compare their value. This
comparison ultimately led him to the choice of some few valuable sorts, one
of which, the "Bellevue de Talavera," still holds its place among the
prominent sorts of wheat cultivated in France. This variety seems to be
really a uniform type, a quality very useful under favourable conditions of
cultivation, but which seems to have destroyed its capacity for further
improvement by selection.
The principle of single-ear sowing, with a view to obtain pure and uniform
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