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.