Darwin and Modern Science (1909)
Edited by A.C. Seward
IV. VARIATION.
By HUGO DE VRIES
Professor of Botany in the University of Amsterdam.
I. DIFFERENT KINDS OF VARIABILITY.
efore 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.[1] 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
strains without further selection, has, until a few years ago, been almost
entirely lost sight of. Only a very few agriculturists have applied it:
among these are Patrick Shirreff ("Die Verbesserung der Getreide-Arten",
translated by R. Hesse, Halle, 1880.) in Scotland and Willet M. Hays
("Wheat, varieties, breeding, cultivation", Univ. Minnesota, Agricultural
Experimental Station, Bull. no. 62, 1899.) in Minnesota. Patrick Shirreff
observed the fact, that in large fields of cereals, single plants may from
time to time be found with larger ears, which justify the expectation of a
far greater yield. In the course of about twenty-five years he isolated in
this way two varieties of wheat and two of oats. He simply multiplied them
as fast as possible, without any selection, and put them on the market.
Hays was struck by the fact that the yield of wheat in Minnesota was far
beneath that in the neighbouring States. The local varieties were Fife and
Blue Stem. They gave him, on inspection, some better specimens,
"phenomenal yielders" as he called them. These were simply isolated and
propagated, and, after comparison with the parent-variety and with some
other selected strains of less value, were judged to be of sufficient
importance to be tested by cultivation all over the State of Minnesota.
They have since almost supplanted the original types, at least in most
parts of the State, with the result that the total yield of wheat in
Minnesota is said to have been increased by about a million dollars yearly.
Definite progress in the method of single-ear sowing has, however, been
made only recently. It had been foreshadowed by Patrick Shirreff, who
after the production of the four varieties already mentioned, tried to
carry out his work on a larger scale, by including numerous minor
deviations from the main type. He found by doing so that the chances of
obtaining a better form were sufficiently increased to justify the trial.
But it was Nilsson who discovered the almost inexhaustible polymorphy of
cereals and other agricultural crops and made it the starting-point for a
new and entirely trustworthy method of the highest utility. By this means
he has produced during the last fifteen years a number of new and valuable
races, which have already supplanted the old types on numerous farms in
Sweden and which are now being introduced on a large scale into Germany and
other European countries.
It is now twenty years since the station at Svalof was founded. During the
first period of its work, embracing about five years, selection was
practised on the principle which was then generally used in Germany. In
order to improve a race a sample of the best ears was carefully selected
from the best fields of the variety. These ears were considered as
representatives of the type under cultivation, and it was assumed that by
sowing their grains on a small plot a family could be obtained, which could
afterwards be improved by a continuous selection. Differences between the
collected ears were either not observed or disregarded. At Svalof this
method of selection was practised on a far larger scale than on any German
farm, and the result was, broadly speaking, the same. This may be stated
in the following words: improvement in a few cases, failure in all the
others. Some few varieties could be improved and yielded excellent new
types, some of which have since been introduced into Swedish agriculture
and are now prominent races in the southern and middle parts of the
country. But the station had definite aims, and among them was the
improvement of the Chevalier barley. This, in Middle Sweden, is a fine
brewer's barley, but liable to failure during unfavourable summers on
account of its slender stems. It was selected with a view of giving it
stiffer stems, but in spite of all the care and work bestowed upon it no
satisfactory result was obtained.
This experience, combined with a number of analogous failures, could not
fail to throw doubt upon the whole method. It was evident that good
results were only exceptions, and that in most cases the principle was not
one that could be relied upon. The exceptions might be due to unknown
causes, and not to the validity of the method; it became therefore of much
more interest to search for the causes than to continue the work along
these lines.
In the year 1892 a number of different varieties of cereals were cultivated
on a large scale and a selection was again made from them. About two
hundred samples of ears were chosen, each apparently constituting a
different type. Their seeds were sown on separate plots and manured and
treated as much as possible in the same manner. The plots were small and
arranged in rows so as to facilitate the comparison of allied types.
During the whole period of growth and during the ripening of the ears the
plots were carefully studied and compared: they were harvested separately;
ears and kernels were counted and weighed, and notes were made concerning
layering, rust and other cereal pests.
The result of this experiment was, in the main, no distinct improvement.
Nilsson was especially struck by the fact that the plots, which should
represent distinct types, were far from uniform. Many of them were as
multiform as the fields from which the parent-ears were taken. Others
showed variability in a less degree, but in almost all of them it was clear
that a pure race had not been obtained. The experiment was a fair one,
inasmuch as it demonstrated the polymorphic variability of cereals beyond
all doubt and in a degree hitherto unsuspected; but from the standpoint of
the selectionist it was a failure. Fortunately there were, however, one or
two exceptions. A few lots showed a perfect uniformity in regard to all
the stalks and ears: these were small families. This fact suggested the
idea that each might have been derived from a single ear. During the
selection in the previous summer, Nilsson had tried to find as many ears as
possible of each new type which he recognised in his fields. But the
variability of his crops was so great, that he was rarely able to include
more than two or three ears in the same group, and, in a few cases, he
found only one representative of the supposed type. It might, therefore,
be possible that those small uniform plots were the direct progeny of ears,
the grains of which had not been mixed with those from other ears before
sowing. Exact records had, of course, been kept of the chosen samples, and
the number of ears had been noted in each case. It was, therefore,
possible to answer the question and it was found that those plots alone
were uniform on which the kernels of one single ear only had been sown.
Nilsson concluded that the mixture of two or more ears in a single sowing
might be the cause of the lack of uniformity in the progeny. Apparently
similar ears might be different in their progeny.
Once discovered, this fact was elevated to the rank of a leading principle
and tested on as large a scale as possible. The fields were again
carefully investigated and every single ear, which showed a distinct
divergence from the main type in one character or another, was selected. A
thousand samples were chosen, but this time each sample consisted of one
ear only. Next year, the result corresponded to the expectation.
Uniformity prevailed almost everywhere; only a few lots showed a
discrepancy, which might be ascribed to the accidental selection of hybrid
ears. It was now clear that the progeny of single ears was, as a rule,
pure, whereas that of mixed ears was impure. The single-ear selection or
single-ear sowing, which had fallen into discredit in Germany and elsewhere
in Europe, was rediscovered. It proved to be the only trustworthy
principle of selection. Once isolated, such single-parent races are
constant from seed and remain true to their type. No further selection is
needed; they have simply to be multiplied and their real value tested.
Patrick Shirreff, in his early experiments, Le Couteur, Hays and others had
observed the rare occurrence of exceptionally good yielders and the value
of their isolation to the agriculturist. The possibility of error in the
choice of such striking specimens and the necessity of judging their value
by their progeny were also known to these investigators, but they had not
the slightest idea of all the possibilities suggested by their principle.
Nilsson, who is a botanist as well as an agriculturist, discovered that,
besides these exceptionably good yielders, every variety of a cereal
consists of hundreds of different types, which find the best conditions for
success when grown together, but which, after isolation, prove to be
constant. Their preference for mixed growth is so definite, that once
isolated, their claims on manure and treatment are found to be much higher
than those of the original mixed variety. Moreover, the greatest care is
necessary to enable them to retain their purity, and as soon as they are
left to themselves they begin to deteriorate through accidental crosses and
admixtures and rapidly return to the mixed condition.
Reverting now to Darwin's discussion of the variability of cereals, we may
conclude that subsequent investigation has proved it to be exactly of the
kind which he describes. The only difference is that in reality it reaches
a degree, quite unexpected by Darwin and his contemporaries. But it is
polymorphic variability in the strictest sense of the word. How the single
constituents of a variety originate we do not see. We may assume, and
there can hardly be a doubt about the truth of the assumption, that a new
character, once produced, will slowly but surely be combined through
accidental crosses with a large number of previously existing types, and so
will tend to double the number of the constituents of the variety. But
whether it first appears suddenly or whether it is only slowly evolved we
cannot determine. It would, of course, be impossible to observe either
process in such a mixture. Only cultures of pure races, of single-parent
races as we have called them, can afford an opportunity for this kind of
observation. In the fields of Svalof new and unexpected qualities have
recently been seen, from time to time, to appear suddenly. These
characters are as distinct as the older ones and appear to be constant from
the moment of their origin.
Darwin has repeatedly insisted that man does not cause variability. He
simply selects the variations given to him by the hand of nature. He may
repeat this process in order to accumulate different new characters in the
same family, thus producing varieties of a higher order. This process of
accumulation would, if continued for a longer time, lead to the
augmentation of the slight differences characteristic of varieties into the
greater differences characteristic of species and genera. It is in this
way that horticultural and agricultural experience contribute to the
problem of the conversion of varieties into species, and to the explanation
of the admirable adaptations of each organism to its complex conditions of
life. In the long run new forms, distinguished from their allies by quite
a number of new characters, would, by the extermination of the older
intermediates, become distinct species.
Thus we see that the theory of the origin of species by means of natural
selection is quite independent of the question, how the variations to be
selected arise. They may arise slowly, from simple fluctuations, or
suddenly, by mutations; in both cases natural selection will take hold of
them, will multiply them if they are beneficial, and in the course of time
accumulate them, so as to produce that great diversity of organic life,
which we so highly admire.
Darwin has left the decision of this difficult and obviously subordinate
point to his followers. But in his Pangenesis hypothesis he has given us
the clue for a close study and ultimate elucidation of the subject under
discussion.
Editor's Note
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.
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