CHAPTER III

VARIATION

The assumption of Darwin and Wallace that variations are haphazard in

origin and indefinite in direction—If these assumptions be not correct

Natural Selection ceases to be the fundamental factor in

evolution—Darwin’s views regarding variation underwent modification—He

eventually recognised the distinction between definite and indefinite

variations, and between continuous and discontinuous variations—Darwin

attached but little importance to either definite or discontinuous

variations—Darwin’s views on the causes of variations—Criticism of

Darwin’s views—Variations appear to occur along certain definite

lines—There seems to be a limit to the extent to which fluctuating

variations can be accumulated—De Vries’ experiments—Bateson on

“discontinuous variation”—Views held by De Vries—Distinction between

continuous and discontinuous variations—The work of De Vries—Advantages

enjoyed by the botanist in experimenting on the making of

species—Difficulties encountered by the animal breeder—Mutations among

animals—The distinction between germinal and somatic variations—The

latter, though not transmitted to offspring, are often of considerable

value to their possessor in the struggle for existence.

Nature of Variation

As we have already seen, the Darwinian theory, unlike that of Lamarck,

does not attempt to explain the origin of variations. It is content with

the fact that variations do occur.

Although Darwin did not try to explain how it is that variation occurs,

and was very guarded in the expressions he used concerning it, he assumed

that variations are indefinite in variety and occur indiscriminately in

all directions, as the following quotations from the _Origin of Species_

will show: “But the number and diversity of inheritable deviations of

structure . . . are endless” (page 14, ed. 1902). “The variations are

supposed to be extremely slight, but of the most diversified nature.” “I

have hitherto sometimes spoken as if the variations so common and

multiform with organic beings under domestication, and in a lesser degree

to those under nature, were due to chance. This, of course, is a wholly

incorrect expression, but it serves to acknowledge plainly our ignorance

of the cause of each particular variation” (page 164).

Wallace is far less guarded in his expressions. On page 82 of his

_Darwinism_ he speaks of “the constant and large amount of variation of

every part in all directions . . . which must afford an ample supply of

favourable variations whenever required.”

The double assumption that variations are for all practical purposes

haphazard in origin and indefinite in direction is necessary if natural

selection is to be the main factor in evolution. For if variations be not

haphazard, if they are definite, if there be a directive force behind

them, like fate behind the classical gods, then selection is not the

fundamental cause of evolution. It can at most effect, not the origin of

species, but the survival of certain species which have arisen as the

result of some other force. Its position is changed; it is no longer a

cause of the origin of new organisms, but a sieve determining which of

certain ready-made forms shall survive. Evidently, then, we shall not be

able to fully understand the evolutionary process until we have

discovered how it is that variations are caused. In other words, we must

go considerably farther than Darwin attempted to do.

Before proceeding to inquire into the true nature of variations, it

behoves us to set forth briefly the ideas of Darwin on the subject. We

shall then be in a position to see how much progress has been made since

the days of that great biologist.

It is not at all easy to discover exactly what were Darwin’s views on the

subject of variation. A perusal of his works reveals contradictions, and

gives one the impression that he himself scarcely knew his own mind upon

the subject. This should not be a matter for surprise.

We must remember that Darwin had to do pioneer work, that he had to deal

with altogether new conceptions. Such being the case, his ideas were of

necessity somewhat hazy; they underwent considerable modification as

fresh facts came to his knowledge.

Definite and Indefinite Variability

Towards the end of his life Darwin recognised that variability is of two

kinds—definite and indefinite. Indefinite variation is indiscriminate

variation in all directions around a mean, variation which obeys what we

may perhaps call the law of chance. Definite variation is variation in a

determinate direction—variation chiefly on one side of the mean. Darwin

believed that these determinate variations were caused by external

forces, and that they are inherited. He thus accepted Lamarckian factors.

“Each of the endless variations,” he writes, “which we see in the plumage

of our fowls, must have had some efficient cause, and if the same causes

were to act uniformly during a long series of generations on many

individuals, all probably would be modified in the same direction.”

But Darwin was always of opinion that this definite variability, this

variability in one direction as the result of some fixed cause, is far

less important, from an evolutionary point of view, than indefinite

variability, that it is the exception rather than the rule, that the

usual result of changed conditions is to let loose a flood of indefinite

variability, that it is almost exclusively upon this that natural

selection acts.

Darwin also recognised that variations differ in degree, even as they do

in kind. He perceived that some variations are much more pronounced than

others. He recognised the distinction between what are now known as

continuous and discontinuous variations. The former are slight departures

from the normal; the latter are considerable deviations from the mean or

mode; great jumps, as it were, taken by nature, as, for example, the pea

and the rose combs of fowls, which were derived from the normal single

comb.

Monstrosities

“At long intervals of time,” wrote Darwin, “out of millions of

individuals reared in the same country and fed on nearly the same food,

deviations of structure so strongly pronounced as to deserve to be called

monstrosities arise, but monstrosities cannot be separated by any

distinct line from slighter variations.” Therefore it is evident that he

regarded the difference between continuous and discontinuous variations

as not one of kind, but merely of degree. To the discontinuous variations

Darwin attached very little importance from an evolutionary point of

view. He looked upon them as something abnormal.

“It may be doubted,” he wrote, “whether such sudden and considerable

deviations of structure such as we occasionally see in our domestic

productions, more especially with plants, are ever permanently propagated

in a state of nature. Almost every part of every organic being is so

beautifully related to its complex conditions of life that it seems as

improbable that any part should have been suddenly produced perfect, as

that a complex machine should have been invented by a man in a perfect

state. Under domestication monstrosities sometimes occur which resemble

normal structures in widely different animals. Thus pigs have

occasionally been born with a sort of proboscis, and if any wild species

of the same genus had naturally possessed a proboscis, it might have been

argued that this had appeared as a monstrosity; but I have as yet failed

to find, after diligent search, cases of monstrosities resembling normal

structures in nearly allied forms, and these alone bear on the question.

If monstrous forms of this kind ever do appear in a state of nature and

are capable of reproduction (which is not always the case), as they occur

rarely and singly, their preservation would depend on unusually

favourable circumstances. They would, also, during the first and

succeeding generations cross with the ordinary form, and thus their

abnormal character would almost inevitably be lost.” But, in a later

edition of the _Origin of Species_, Darwin seems to contradict the above

assertion: “It should not, however, be overlooked that certain rather

strongly marked variations, which no one would rank as mere individual

differences, frequently recur owing to a similar organisation being

similarly acted on—of which fact numerous instances could be given with

our domestic productions. In such cases, if the varying individual did

not actually transmit to its offspring its newly acquired character, it

would undoubtedly transmit to them, as long as the existing conditions

remained the same, a still stronger tendency to vary in the same manner.

There can also be little doubt that the tendency to vary in the same

manner has often been so strong that all the individuals of the same

species have been similarly modified without the aid of any form of

selection. Or only a third, fifth, or tenth part of the individuals may

have been thus affected, of which fact several instances could be given.

Thus Graba estimates that about one-fifth of the guillemots in the Faroe

islands consist of a variety so well marked, that it was formerly ranked

as a distinct species under the name _Uria lacrymans_. In cases of this

kind, if the variation were of a beneficial nature, the original form

would soon be supplanted by the modified form, through the survival of

the fittest.” Here we seem to have a plain statement of the origin of new

forms by mutation.

Minute Variations

Again, we read (page 34): “Some variations useful to him (_i.e._ man)

have probably arisen suddenly, or by one step; many botanists, for

instance, believe that the fuller’s teasel, with its hooks, which cannot

be rivalled by any mechanical contrivance, is only a variety of the wild

Dipsacus; and this amount of change may have suddenly arisen in a

seedling. This is known to be the case with the turnspit dog.”[2] But, as

we have already said, Darwin at no time attached much importance to these

jumps made by nature as a factor in evolution. He pinned his faith to the

minute, indefinite variations which he believed could be piled up, one

upon another, so that, if allowed sufficient time, either nature or the

human breeder could, by a continued selection of these minute variations,

call into being any kind of organism. The importance of selection, he

writes, “consists in the great effect produced by the accumulation in one

direction, during successive generations, of differences absolutely

inappreciable by an uneducated eye” (page 36). On page 132 he writes: “I

can see no limit to the amount of change, to the beauty and complexity of

the coadaptations between all organic beings . . . which may have been

effected[3] in the long course of time by nature’s power of selection.”

He expressly states, on page 149, that he sees no reason to limit the

process to the formation of genera alone.

Although the theory of natural selection does not attempt to explain the

causes of variation, Darwin paid some attention to the subject. He

believed that both internal and external causes contribute to variation,

that variations tend to be inherited whether the result of causes within

the organism or outside it. He believed that the inherited effect of use

and disuse was a cause of variation, and cited, as examples, the lighter

wing-bones and heavier leg-bones of the domestic duck and the drooping

ears of some domestic animals. He supposed that animals showed a greater

tendency to vary when under domestication than when in their natural

state, attributing the supposed greater variability to the excess of food

received, and the changed conditions of the life of domestic animals.

Nevertheless, he was fully alive to the fact that “nearly similar

variations sometimes arise under, as far as we can judge, dissimilar

conditions; and, on the other hand, dissimilar variations arise under

conditions which appear to be nearly uniform.” In other words, the nature

of organisms appeared to Darwin to be a more important factor in the

origin of variations than external conditions. Evidence of this is

afforded by the fact that some animals are more variable than others.

Finally, he frankly admitted how great was his ignorance of the causes of

variability. Variability is, he stated, governed by unknown laws which

are infinitely complex.

Lines of Variation

It will be convenient to deal with each of Darwin’s main ideas on

variation separately, and to consider to what extent they seem to require

modification in the light of later research.

Firstly, Darwin believed that variations arise in what appears to be a

haphazard manner, that they occur in all directions, and seem to be

governed by the same laws as chance. It is our belief that we are now in

a position to make more definite statements regarding variation than

Darwin was able to.

Biologists can now assert definitely that variations do not always occur

equally in all directions. The results of many years of the efforts of

practical breeders demonstrate this. These men have not been able to

produce a green horse, a pigeon with alternate black and white feathers

in the tail, or a cat with a trunk, for the simple reason that the

organisms upon which they operated do not happen to have varied in the

required direction. It may perhaps be objected that breeders have no

desire to produce such forms; had they wished to do so, they would

probably have succeeded. To this objection we may reply that they have

not managed to produce many organisms, which would be highly desirable

from a breeder’s point of view, as, for example, a blue rose, hens that

lay brown eggs but do not become broody at certain seasons of the year,

or a cat that cannot scratch.

As Mivart well says, on page 118 of his _Genesis of Species_, “Not only

does it appear that there are barriers which oppose change in certain

directions, but that there are positive tendencies to development along

certain special lines. In a bird which has been kept and studied like the

pigeon, it is difficult to believe that any remarkable spontaneous

variations would pass unnoticed by breeders, or that they would not have

been attended to and developed by some fancier or other. On the

hypothesis of indefinite variability, it is then hard to say why pigeons

with bills like toucans, or with certain feathers lengthened like those

of trogons, or those of birds of paradise, have never been produced.”

There are certain lines along which variation seems never to occur. Take

the case of the tail of a bird. Variable though this organ be, there are

certain kinds of tail that are seen neither in wild species nor

domesticated races. A caudal appendage, of which the feathers are

alternately coloured, occurs neither in wild species nor in artificial

breeds. For some reason or other, variations in this direction do not

occur. Similarly, with the exception of one or two of the “Noddy” terns,

whenever a bird has any of its tail feathers considerably longer than the

others, it is always the outer pair or the middle pair that are so

elongated. It would thus appear that variations in which the other

feathers are especially lengthened do not usually occur. The fact that

they are elongated in two or three wild species is the more significant,

because it shows that there is apparently nothing inimical to the welfare

of a species in having, say, the third pair of tail feathers from the

middle exceptionally prolonged.

Breeders’ Boasts

This is a most important point, and one which seems to be ignored by the

majority of scientific men, who appear to be misled by the boastful talk

of certain successful breeders. Thus, on page 29 of the _Origin of

Species_, Darwin quotes, with approval, Youatt’s description of selection

as “the magician’s wand, by means of which he may summon into life

whatever form and mould he pleases.” Darwin further cites Sir John

Sebright as saying, with regard to pigeons, that he would “produce any

given feather in three years, but it would take him six years to obtain

head and beak.”

If it were possible absolutely to originate anything by selection,

horticulturists would almost certainly ere this have produced a pure

black flower. The fact that not a single mammal exists, either in nature

or under domestication, with scarlet, blue, or green in its hair, appears

to show that, for some reason or other, mammals never vary in any of

these directions.

The fact that so few animals have developed prehensile tails seems to

indicate that variation does not often occur in that direction, for

obviously a prehensile tail is of the very greatest utility to its

possessor; so that there can be little room for doubt that it would be

seized upon and preserved by natural selection, whenever it occurred.

As E. H. Aitken very truly says, “so early and useful an invention

should, one would think, have been spread widely in after time; but there

appears to be some difficulty in developing muscles at the thin end of a

long tail, for the animals that have turned it into a grasping organ are

few and are widely scattered. Examples are the chameleon among lizards,

our own little harvest mouse, and, pre-eminent among all, the American

monkeys” (_Strand Magazine_, Nov. 1908).

Even as there are many variations which seem never to occur in nature, so

are there others which occur so frequently that they may be looked for in

any species. Albinistic forms appear now and again in almost every

species of mammal or bird; while melanistic sports, although not so

common, are not by any means rare.

Every complete manual on poultry gives for each breed a note of the

faults which constantly appear, and which the fancier has to watch

carefully for and guard against. The fact that these “faults” occur so

frequently in each breed shows how strong is the tendency to vary in

certain definite directions. It is true that some of these faults are in

the nature of reversions, as, for example, the appearance of red hackles

in the cocks of black breeds of poultry. On the other hand, some

certainly are not reversions, such as the appearance of a white ring in

the neck of the female of the Rouen duck, which should resemble the

Mallard as regards the plumage of the neck. Again, the tendency of Buff

Orpingtons to assume white in the wings and tail must be regarded as a

variation which is not in the nature of a reversion. In short, the

efforts of all breeders are largely directed to fighting against the

tendencies which animals display towards variation in certain directions.

Albinistic Variations

This tendency to vary in the direction of whiteness may account for many

of the white markings which occur in nature, as, for example, the white

tails of the Sea Eagle (_Haliaetus albicilla_) the Nicobar Pigeon

(_Caloenas nicobarica_), and many hornbills. Provided that such

variations are not too great a handicap to their possessors in the

struggle for existence, natural selection will allow them to persist.

It was the belief of Linnæus, based on experience, that every blue or

red-coloured flower is likely to produce a white variety, hence he held

that it is not safe to trust to colour for the identification of a

botanical species.

On the other hand, white flowers are not likely to produce red varieties,

and we believe we may positively assert that they never produce a blue

sport. Similarly, white animals appear not to give rise to colour

varieties.

We are never surprised to find that an ordinary upright plant produces as

a sport or mutation a pendulous, or fastigiate form. These aberrant

varieties, be it noted, occur in species which belong to quite different

orders.

De Vries points out that laciniated leaves appear in such widely

separated trees and shrubs as the walnut, the beech, the hazel-nut, and

the turnip.

Another example of the definiteness of variation is furnished by what

Grant Allen calls the “Law of Progressive Colouration” of flowers.

On pp. 20, 21 of _The Colours of Flowers_, he writes, “All flowers, as we

know, easily sport a little in colour. But the question is, do their

changes tend to follow any regular and definite order? Is there any

reason to believe that the modification runs from any one colour toward

any other? Apparently there is. . . . All flowers, it would seem, were in

their earliest form yellow; then some of them became white; after that a

few of them grew to be red or purple; and finally a comparatively small

number acquired the various shades of lilac, mauve, violet, or blue.”

Over-development

So among animals there are many colour patterns and structures that

appear in widely different genera, as, for example, the magpie colouring

in birds. With this phenomenon we shall deal more fully when speaking of

animal colouration. There is certainly no small amount of evidence which

seems to indicate that, from some cause or other, an impetus has been

given to certain organs to develop along definite lines. The reduction of

the number of digits in several mammalian families which are not nearly

related is a case in point. This phenomenon is, as Cope points out,

observed in Marsupials, Rodents, Insectivores, Carnivores, and Ungulates.

He, being a Lamarckian, ascribes this to the inherited effects of use.

Wallaceians attribute it solely to the action of natural selection. The

assumption of a growth-force or tendency for the development of one digit

at the expense of the others, would explain the phenomenon equally well.

And it is significant that many palæontologists are believers in some

kind of a growth-force. In the case of certain extinct animals we seem to

have examples of the over-development of organs. “Palæontology,” writes

Kellog on p. 275 of his _Darwinism To-day_, “reveals to us the one-time

existence of animals, of groups of animals, and of lines of descent,

which have had characteristics which led to extinction. The unwieldiness

of the giant Cretaceous reptiles, the fixed habit of life of the

crinoids, the coiling of the ammonities and the nautili, the gigantic

antlers of the Irish stag—all these are examples of development along

disadvantageous lines, or to disadvantageous degrees. The statistical

studies of variation have made known numerous cases where the slight, as

yet non-significant (in a life-and-death struggle) variation in pattern

of insects, in dimensions of parts, in relative proportions of

superficial non-active areas, are not fortuitous, that is, do not occur

scattered evenly about a mean or mode according to the law of error, but

show an obvious and consistent tendency to occur along certain lines, to

accumulate in certain directions.”

It seems to us that the only proper attitude to adopt in the present

state of our knowledge is, not to call in to our aid an unknown

growth-force, but simply to say that there is evidence to show that

variations frequently occur along certain definite lines only.

Speed of Racehorses

Darwin’s second assumption was that there is no limit to which variations

may be accumulated in any direction; that by adding one minute variation

to another through countless generations new species, new genera, new

families may arise. This assumption, if applied to continuous or

fluctuating variations, seems opposed to facts. All the evidence

available goes to show that there is a definite limit to which minute

variations can be accumulated in any given direction. No one has

succeeded in breeding a dog as large as a horse, or a pigeon with a beak

as long as that of a snipe. In the case of racehorses, which have been

selected so carefully through a long period of time, we seem to have

reached the limit of speed which can be attained by the multiplication of

insignificant variations. We do not wish to dogmatise, but we believe

that of late years there has not been any material increase in the speed

of our racehorses.

Mr S. Sidney says, on page 174 of _Cassell’s Book of the Horse_: “As far

as form went (_pace_ Admiral Rous), the British racehorse had reached

perfection in 1770, when ‘Eclipse’ was six years old.” He quotes the

measurements of the skeleton of “Eclipse” in the Museum of the Royal

College of Surgeons as evidence of this. All the efforts of breeders,

then, have failed appreciably to improve the form of the British

racehorse in the course of over a century and a quarter.

Experiments of De Vries

De Vries has made some important experiments with a view to determining

whether or not there is a limit to the amount of change which can be

induced by the selection of fluctuating or continuous variations as

opposed to mutations. “I accidentally found,” he writes, on page 345 of

_Species and Varieties: their Origin by Mutation_, “two individuals of

the ‘five-leaved’ race (of clover); by transplanting them into my garden

I have isolated them and kept them free from cross-fertilisation with the

ordinary type. Moreover, I brought them under such conditions as are

necessary for the full development of their character; and last, but not

least, I have tried to improve their character as far as possible by a

very rigid and careful selection. . . . By this method I brought my

strain within two years up to an average of nearly 90 per cent. of the

seedlings with a divided primary leaf (such seedlings averaging five

leaves in the adult). . . . This condition was reached by the sixth

generation in the year 1894, and has since proved to be the limit, the

figures remaining practically the same through all the succeeding

generations. . . . I have cultivated a new generation of this race nearly

every year since 1894, using always the strictest selection. This has led

to a uniform type, but has not been adequate to produce further

improvement.” Similarly, De Vries found in the bulbous buttercup

(_Ranunculus bulbosus_) a strain varying largely in the number of petals;

therefore he tried by means of continuous selection of those flowers

having the largest number of petals to produce a double flower, but was

not able to do so. He succeeded in evolving a strain with an average

number of nine petals, some individuals having as many as twenty or

thirty; but even by breeding only from these last he could not increase

the average number of petals in any generation beyond nine. This was the

limit to be obtained by the most rigorous selection of fluctuating

variations.

Selection, based on fluctuating variation, does not, asserts De Vries,

conduce to the production of improved races. “Only temporary

ameliorations are obtained, and the selection must be made in the same

manner every year. Moreover, the improvement is very limited, and does

not give any promise of further increase.” Notwithstanding prolonged

efforts, horticulturists have not yet succeeded in breeding a biennial

race of either beetroots or carrots that does not continually give rise

to useless annual forms. Writing of the beet, De Vries says useless

annual varieties “are sure to return each year. They are ineradicable.

Every individual is in the possession of this latent quality, and liable

to convert it into activity as soon as the circumstances provoke its

appearance, as is proved by the increase of annuals in the early

sowings”—that is to say, in circumstances favourable to the annual

variety.

It will be urged perhaps that these experiments, which seem to show that

there is a limit to which a species can be modified by the accumulation

of fluctuating variations, cannot have been properly carried out, because

all the various breeds of pigeons and other domestic animals clearly show

that extraordinary differences not only can, but have actually been

produced by the selection of such variations. This objection is based

upon the assumption that breeders have in the past dealt only with

fluctuating variations. This assumption does not appear to be justified.

It is exceedingly probable that most, if not all, the varieties of

domesticated animals have originated in mutations. Take, for instance,

the modern turbit pigeon; this has been derived from the old Court-bec,

described and figured over two centuries ago by Aldrovandus.

De Vries goes so far as to assert that the various races of pears are all

mutations; that each distinct flavour is a mutation, and that it is

impossible to produce a new flavour by selecting fluctuating variations.

Thus it would appear that in every case of the production of a new breed

a mutation has occurred which has attracted the fancy of some breeder,

and he has seized upon this and perpetuated it.

All the evidence available tends to show that there is a limit—and one

which is quickly reached—to the amount of change that can be produced by

the selection of fluctuating or continuous variations. We, therefore,

seem driven to the belief that evolution is based on the kind of

variation which Professor Bateson terms “discontinuous variation” and

Professor De Vries calls “mutation.”

Bateson on Variation

As long ago as 1894 Bateson published his _Materials for the Study of

Variation_, in which he set forth a large number of cases of

discontinuous variation which he had collected. He pointed out that

species are discontinuous, that they are sharply separated one from

another, whereas “environments often shade into one another and form a

continuous series.” How, then, he asked, if variations are minute and

continuous, have these discontinuous species arisen? May not variation

prove to be discontinuous, and thus make it clear why species are

discontinuous?

On page 15 of the above-cited work we find: “The preliminary question,

then, of the degree of continuity with which the process of evolution

occurs has never been decided. In the absence of such a decision, there

has nevertheless been a common assumption, either tacit or expressed,

that the process is a continuous one. The immense consequence of a

knowledge of the truth as to this will appear from a consideration of the

gratuitous difficulties which have been introduced by this assumption.

Chief among these is the difficulty which has been raised in connection

with the building up of new organs in their initial and imperfect stages,

the mode of transformation of organs, and, generally, the selection and

perpetuation of minute variations. Assuming, then, that variations are

minute, we are met by this familiar difficulty. We know that certain

devices and mechanisms are useful to their possessors; but from our

knowledge of natural history we are led to think that their usefulness is

consequent on the degree of perfection in which they exist, and that if

they were at all imperfect, they would not be useful. Now it is clear

that in any continuous process of evolution such stages of imperfection

must occur, and the objection has been raised that natural selection

cannot protect such imperfect mechanisms so as to lift them into

perfection. Of the objections which have been brought against the theory

of natural selection this is by far the most serious.”

Bateson further pointed out that chemical compounds are not continuous,

that they do not merge gradually each into the next, and suggested that

we might expect a similar phenomenon in the organic world.

Elsewhere he says: “Let the believer in the efficacy of selection

operating on continuous fluctuations try to breed a white or a black rat

from a pure strain of black-and-white rats, by choosing for breeding the

whitest or the blackest; or to raise a dwarf sweet pea from a tall race

by choosing the shortest. It will not work. Variation leads and selection

follows.”

Work of Bateson and De Vries

But Bateson’s views fell upon stony ground, because zoologists are mostly

men of theory and not practical breeders. They laboured under the

delusion that mutations or “sports” are rare in nature, and that when

these do happen to occur they must of necessity be swamped by

inter-crossing.

However, the discovery of the Abbé Mendel’s account of his experiments on

breeding mongrel sweet peas has opened the eyes of many zoologists, so

that they have at last learned what practical breeders have known for

untold years—namely, that sports have a way of perpetuating themselves.

Moreover, Mendel was able to give a theoretical explanation of his

discoveries, with the result that the believers in discontinuous

variation have largely increased in number of late.

While we are unable to see eye to eye with Professor Bateson in all

things, we gladly recognise the immense value of his work. Had his

statements in 1894 received the attention they merited, zoological theory

would to-day be considerably more advanced than it actually is.

Professor De Vries has gone farther than Bateson, having engrafted upon

the Darwinian hypothesis the theory of mutations. He has done no small

amount of experimental work, and has undoubtedly thrown much new light on

the ways in which species arise. He is purely a botanist, so that he

argues only from plants. Nevertheless, we believe that some of his

conclusions are applicable to animals. We are far from accepting his

theory of mutations _in toto_. We are, however, convinced that he, like

Bateson, is on the right track. There can be no doubt that a great many

new forms have originated suddenly, by jumps, and not by imperceptibly

slow degrees. Before giving a list of the names of some of the races,

both plant and animal, which appear to have come into existence suddenly,

it will be of advantage to consider for a little some of the more

important conceptions of De Vries.

Varieties and Elementary Species

That eminent botanist, as we have already seen, insists on the

distinction between fluctuating variations and mutations. The former

correspond, for all practical purposes, to the continuous variations of

Bateson, and the latter seem to be equivalent to his discontinuous

variations.

According to De Vries, all plants display fluctuating variation, but only

a small percentage exhibit the phenomenon of mutation. The most daring of

his conceptions is, that the history of every species is made up of

alternating periods of inactivity, when only fluctuating variations

occur, and of activity when “swarms of species” are produced by mutation,

and of these only a few at the most survive; natural selection, which De

Vries likens to a sieve, determining which shall live and which shall

perish.

As we have seen, De Vries does not believe that new species can arise by

the accumulation of fluctuating variations. By means of these the race

may be greatly improved, but nothing more can be accomplished. These

variations follow Quetelet’s law, which says that, for biological

phenomena, deviations from the average comply with the same laws as the

deviations from the average in any other case, if ruled by chance alone.

Very different in character are mutations. By means of these, new forms,

quite unlike the parent species, suddenly spring into being. Mutations

are said by De Vries to be of two kinds—those that produce varieties and

those which result in new elementary species.

According to De Vries, those species of plants which are in a state of

mutation (he refers to the species of the systematic botanists) are of a

composite nature, being made up of a collection of varieties and

elementary species. His conception of a variety is a plant that differs

from the parent plant in the loss or suppression of one or more

characters, while an elementary species differs from the parent form in

the possession of some new and additional character. But we will allow

him to speak for himself: “We can consider (page 141 _Species and

Varieties_) the following as the principal difference between elementary

species and varieties: that the first arise by the acquisition of

entirely new characters, and the latter by the loss of existing

qualities, or by the gain of such peculiarities as may already be seen in

other allied species. If we suppose elementary species and varieties

originated by sudden leaps and bounds, or mutations, then the elementary

species have mutated in the line of progression, some varieties have

mutated in the line of retrogression, while others have diverged from the

parental types in a line of digression or in the way of repetition. . . .

The system (of the vegetable kingdom) is built up of species; varieties

are only local and lateral, never of real importance for the whole

structure.”

De Vries asserts that these elementary species, when once they arise,

breed true, and show little or no tendency to revert to the ancestral

form. We can, says De Vries, ascertain only by experiment which plants

are in the mutating state and which are not. The great majority, however,

are not at present in the mutating state.

Mutations

The distinction between fluctuating variation and mutation has been

roughly illustrated by the case of a solid block of wood having a number

of facets, on one of which it stands. If the block be tilted slightly it

will, when the force that has tilted it is removed, return to its old

position. Such a gentle tilt may be compared to a fluctuating variation

in an organism. If, however, the block be tilted to such an angle that

when left to itself the block does not return to its old position, but

tips over and comes to rest on another facet, we have a representation of

the kind of change indicated by a mutation.

The analogy is far from perfect, for it makes it appear that the smallest

mutation must of necessity involve a departure from the normal type more

considerable than that of the largest fluctuating variation. Now,

although mutations ordinarily consist in considerable deviations from the

mean or mode of the type, while continuous variations are usually minute

deviations, it sometimes happens that the extreme fluctuations are more

considerable than some mutations. Hence “fluctuating” describes this

latter kind of variation more accurately than “continuous” does.

The test, then, of a mutation is not so much the amount of deviation as

the degree in which it is inherited. Mutations show no tendency to a

gradual return to the mean of the parent species; fluctuating variations

do display such a tendency. A mutation consists, as M. E. East says, in

the production of a new mode or centre for linear fluctuation; it is, as

it were, a shifting of the centre of gravity; the centre about which

those fluctuations which we call continuous variations occur.

As it is of considerable importance thoroughly to grasp the true nature

of mutations or discontinuous variations, and as some writers do not

appear to realise wherein lies the essential difference between the two

kinds of variation, we will, at the risk of appearing tedious, give a

further illustration. Let A be a species of bird of which the average

length of the wing is 20 inches, and let us suppose that individuals

belonging to that species occur in which the length of the wing varies as

much as 3 inches each side of the mean; thus it is possible to find

individuals of this species with a wing as short as 17 inches, or as long

as 23 inches. Let B be another species of which the average length of the

wing is 17 inches, and let us suppose that a 3-inch variation on each

side of the mean be found to occur. Individuals belonging to species B

will occur which have a wing as short as 14 inches, or as long as 20

inches. Thus some individuals of the short-winged species will have

longer wings than certain individuals of the long-winged species.

Similarly, certain individuals of a species which display a mutation may

show less deviation from the mean than some individuals showing a very

pronounced fluctuating variation. In other words, even as by measuring

the length of wing in the above example it was not always possible to say

whether a given individual belonged to species A or B, so is it not

always possible to say by looking at an individual that shows a

considerable departure from the mean whether that departure is due to a

mutation or a fluctuating variation.

Law of Regression

It is only by watching the effect of the peculiarity on the offspring of

its possessor that we are able to determine the nature of the variation.

Where the peculiarity is due to a fluctuating variation the offspring

will display the peculiarity in a diminished degree; but if the

peculiarity be due to a mutation, the offspring are likely to display it

in as marked a degree as the parent.

Fritz Müller and Galton conducted independently enquiries into the amount

of the regression shown by the progeny of parents which have deviated

from the average by fluctuating variation.

Müller experimented with Indian corn; Galton with the sweet pea.

Each found that where the deviation of the parents is represented by the

figure 5, that of their offspring is usually 2, that is to say, the

deviation they display is, on the average, less than half that of their

parents.

Applying this rule to the hypothetical case given above, if two

individuals of species A having a length of wing of 20 inches be bred

together, their offspring will, on an average, have a length of wing of

20 inches, since neither parents showed any deviation from the mean. On

the other hand, the offspring of 20-inch-wing individuals of species B

would show, on an average, a length of wing of only about 18¼ inches.

They tend to return to that mode from which their parents had departed.

But suppose that the deviation of the parents in this case had been due,

not to fluctuating variation, but to a mutation; this would mean that,

owing to some internal change in the egg that produced each parent, 20

inches became the normal length of wing; that the normal length of wing

had suddenly shifted from 17 inches to 20 inches.

The result of this would be that their offspring would have on an average

a wing-length of 20 inches instead of 18¼ inches, that the centre of

variation as regards length of wing had suddenly shifted from 17 to 20,

that, in future, all fluctuating variations would occur on either side of

20 inches, instead of on either side of 17 inches as heretofore.

Thus a variation is a fluctuating one or a mutation according as it does

or does not obey Galton’s Law of Regression.

De Vries’s Dictum

De Vries says that it is of the essence of mutations that they are

completely inherited. This statement, although substantially true, fails

to take into consideration the factor of fluctuating variation. For

example, in the above instance if the two individuals of species B had

mutated into forms with a 20-inch wing, their offspring will nevertheless

vary _inter se_, some of them will have wings shorter than 20 inches and

others wings more than 20 inches in length. But the average wing-length

of the offspring of the two mutating individuals will be 20 inches.

So much, then, for the practical difference between a mutation and a

fluctuating variation. In Chapter V. we shall discuss the possible causes

of the difference. By way of anticipation we may say that the suggestion

we shall make is that a mutation is due to some rearrangement in the

particles which represent that part of the organism in the fertilised

egg, whereas a fluctuating variation is caused by variations in the

particles themselves.

De Vries, it should be noted, bases his theory largely on experimental

evidence. His dictum is “the origin of species is an object of

experimental observation.” He has, we consider, proved conclusively that

among plants mutations sometimes occur, and, further, that in a mutating

plant the same mutation tends to occur again and again. This latter is a

most important fact, because it goes some way towards overcoming the

difficulty urged by Darwin that isolated sports must be swamped by

continual crossing with the normal type. If mutations arise in swarms, as

De Vries asserts they do, then any particular mutation is likely, sooner

or later, to cross with a similar mutation and so be able to perpetuate

itself.

Mutating Plants

The classical example of a mutating plant is the evening primrose of the

species _Oenothera lamarckiana_. This is described by De Vries as a

stately plant, with a stout stem, attaining often a height of 1.6 metres

or more. The flowers are large and of a bright yellow colour, attracting

immediate attention, even from a distance. “This striking species,” he

writes, in _Species and Varieties_ (p. 525), “was found in a locality

near Hilversum, in the vicinity of Amsterdam, where it grew in some

thousands of individuals. Ordinarily biennial, it produces rosettes in

the first, and stems in the second year. Both the stems and the rosettes

were seen to be highly variable, and soon distinct varieties could be

distinguished among them.

“The first discovery of this locality was made in 1886. Afterwards I

visited it many times, often weekly or even daily, and always at least

once a year up to the present time. This stately plant showed the

long-sought peculiarity of producing a number of new species every year.

Some of them were observed directly in the field, either as stems or

rosettes. The latter could be transplanted into my garden for further

observation, and the stems yielded seeds to be sown under like control.

Others were too weak to live a sufficiently long time in the field. They

were discovered by sowing seed from indifferent plants of the wild

locality in the garden. A third and last method of getting still more new

species from the original strain was the repetition of the sowing

process, by saving and sowing the seed which ripened on the introduced

plants. These various methods have led to the discovery of over a dozen

new types, never previously observed or described.” Some of these De

Vries regards as varieties, in the sense in which he uses the words;

others, he maintains, are real progressive species, some of which are

strong and healthy, others weaker and apparently not destined to be

successful. All these types proved absolutely constant from seed.

“Hundreds of thousands of seedlings may have arisen, but they always come

true and never revert to the original _O. lamarckiana_ type. But some of

them, however, are, like their parent form, liable to mutations.” The

case of the evening primrose is by no means an isolated one. De Vries

cites several other instances of plants in a mutating state. “The common

poppy,” he says (p. 189), “varies in height, in colour of foliage and

flowers; the last are often double or laciniated. It may have white or

bluish seeds, the capsules may open themselves or remain closed, and so

on. But every single variety is absolutely constant, and never runs into

another when the flowers are artificially pollinated and the visits of

insects excluded.” Similarly the garden carnation sometimes gives rise to

the wheat-ear form. “In this variety,” writes De Vries (p. 228), “the

flower is suppressed, and the loss is attended by a corresponding

increase in the number of pairs of bracts. This malformation results in

square spikes, or somewhat elongated heads, consisting only of the

greenish bracts. As there are no flowers, the variety is quite sterile,

and, as it is not regarded by horticulturists as an improvement on the

ordinary bright carnations, it is seldom multiplied by layering.

Notwithstanding this it appears from time to time, and has been seen in

different countries and at different periods, and what is of great

importance for us, in different strains of carnations. Though sterile,

and obviously dying out as often as it springs into existence, it is

nearly two centuries old. It was described in the beginning of the

eighteenth century by Volckamer, and afterwards by Jaeger, De Candolle,

Weber, Masters, Magnus, and many other botanists. I have had it twice at

different times and from different growers.” Similarly, the long-headed

green dahlia arose twice over some years ago in the nursery of Messrs

Zocher & Co.

Further, the peloric Toad-flax (_Linaria vulgaris peloria_) is, De Vries

informs us, “known to have originated from the ordinary type at different

times and in different countries under more or less divergent

conditions.” And, as this variety is wholly barren, it must in each

instance have had an independent origin. Lastly, the purple beech seems

to be a mutation which has originated at least three times over.

Mutation Theory Criticised

Every one interested in biological theory should read both _Species and

Varieties_ and _Plant Breeding_ by De Vries, works which are of

incalculable value to the horticulturist and agriculturist as well as to

the biologist.

While not wishing to detract in any way from the truly splendid work done

by De Vries, we feel constrained to bring several charges against him.

Firstly, he suffers from the complaint that seizes nine out of ten

originators of new theories. He pushes his theory to extreme lengths; he

allows his imagination to run away with him. We do not think that on the

evidence available he is justified in asserting that every species passes

through alternating periods of comparative quiescence and periods in

which it throws off, as mutations, swarms of elementary species. He is

justified in asserting that discontinuous variation is by no means an

uncommon phenomenon, but further than this it does not seem safe to go at

present.

Secondly, he ought to lay more stress on the fact that _Oenothera

lamarckiana_ is a plant which does not appear to be known in the wild

state, and that it is therefore possibly a hybrid plant, and the

so-called elementary species which it gives off may be merely the

varieties out of which it has been built up. Boulenger and Bailey have

both studied this plant, and they have not been able to witness all the

mutations of which De Vries speaks, so that the former says, “The fact

that _Oenothera lamarckiana_ was originally described from a garden

flower, grown in the Paris _Jardin des Plantes_, and that, in spite of

diligent search, it has not been discovered wild anywhere in America,

favours the probability that it was produced by crossing various forms of

the polymorphic _Oenothera biennis_, which had been previously introduced

in Europe.”

Definition of a Species

It has further been objected that, even if these various forms which

Lamarck’s evening primrose throws off are true mutations, they ought not

to be called new species, for they do not differ sufficiently from the

parent species to deserve the name of new species. The reply to this

criticism is that De Vries asserts that mutations produce new elementary

species, which are not the same things as new species in the ordinary

sense of the term. Most Linnæan species differ from one another to a far

greater extent than do elementary species. It seems to us quite plain

that new species arise, not by a single mutation, but by two or three

successive mutations which occur in various parts of an organism.

First arises a well-marked variety, by a single mutation. Subsequent

mutations follow, so that a distinct race is produced. And, finally,

fresh mutations occur, so that a new species is eventually produced.

What De Vries calls an elementary species the majority of systematists

would call a well-marked variety.

We may take this opportunity of remarking that the definition of a

species is one on which naturalists seem unable to agree.

So vast is the field of biology, that now-a-days biologists are compelled

to specialise to some extent. Thus we have botanists, ornithologists,

those who devote themselves to the study of mammals, those who confine

themselves to reptiles, or insects, or fishes, or crustaceans, or

bacteria, etc.

Now each class of systematists has its own particular criterion of what

constitutes a species. Ornithologists do not seem very exacting. Most of

them appear to consider a constant difference of colour sufficient for

the formation into a species of the birds that display such a variation.

Those who study reptiles, on the other hand, do not allow that a mere

difference in colour is sufficient to promote its possessor to specific

rank. Into these nice questions we cannot enter. For our purpose a

species is a group of individuals that differ from all other individuals

in displaying certain well-marked and tolerably constant characters,

which they transmit to their offspring.

Our contention, then, is that new species, in the ordinarily-accepted use

of the term, do not arise as a rule by one sudden bound (although they

may sometimes do so), but are the result of the accumulation of several

mutations or discontinuous variations. Some of these mutations are

exceedingly well marked, while others are so small as to be

indistinguishable from the more extreme fluctuating variations. Before

passing on to consider some cases of well-marked mutations which have

occurred among animals and plants, we should like to take this

opportunity of pointing out that as regards experiments in evolution the

botanist is far more favourably situated than the zoologist.

The botanist is able to reproduce many species vegetatively, _e.g._ by

cuttings, and is thus easily able to multiply examples of mutation. He

can also reproduce the great majority of plants by self-fertilisation,

and so experiences no difficulty in “fixing” a new form. Again, plants

are far easier to control than animals; as a rule they can be

transplanted without any impairment of their capacity for breeding.

Moreover, they produce a greater number of offspring than the most

prolific of the higher animals. The animal breeder is thus at an obvious

disadvantage as compared with the horticulturist. It is only with great

difficulty that he can fix the mutations which appear in his stock.

“Scatliff Strain” of Turbit

The history of the production of the “Scatliff strain” of turbit affords

a good example of the kind of difficulties that confront the breeder.

Pigeon fanciers require that the ideal turbit shall have, among other

things, an unbroken “sweep,” that is to say the line of the profile from

the tip of the beak to the back of the head should be the arc of a

circle. As a rule this line is broken by the overgrowth of the wattle at

the base of the beak. Mr Scatliff, however, has succeeded in breeding a

strain which possesses the required description of profile.

“In the year 1895,” writes Mr H. P. Scatliff on page 25 of _The Modern

Turbit_, “I visited Mr Houghton’s lofts and purchased three or four extra

stout and short-beaked stock birds. . . . The following year I mated one

of these to one of my own black hens, and reared one of the most

successful show birds ever bred, viz. ‘Champion Ladybird,’ a black hen. .

. . Most of the leading judges and many turbit breeders remarked upon

this hen’s wonderful profile, which seemed to improve as she got older

instead of getting worse, as is usual in rather coarse-wattled birds. I,

too, had remarked this, and it opened my eyes to a point in turbit

breeding which I had never heard mentioned by any turbit judges or

breeders, and which I believe I am now pointing out for the first time in

print, viz. that the feathers over her beak wattle which formed her front

_grew from the top and right to the front of her wattle, and not from

slightly behind_, as in almost every other turbit of her day; thus, as

the wattle developed and grew coarser, the front became more developed,

and made her head larger without in any way spoiling the sweep of the

profile.

“The same year ‘Ladybird’ was bred I bred eight others from the same

pair, and with one exception all turned out to be hens. There was only

one other hen, however (a dun), that had this same point, but in a lesser

degree than ‘Ladybird,’ and from these two hens nearly all my blacks, and

several of my blues are descended.”

[Illustration: A TURBIT BELONGING TO MR. H. P. SCATLIFF]

Mr Scatliff, having “spotted” this point, looked about him for another

bird having the peculiarity, with the object, if possible, of fixing the

same in his strain. He discovered this point in a pigeon belonging to Mr

Johnston of Hull, and purchased the bird for £20. But it died in the

following spring without producing for Mr Scatliff a single young one.

The next year Scatliff found that a bird belonging to a Mr Brannam had

the required peculiarity and so purchased him for £20. But that cock,

too, died before anything was bred from him. Nothing daunted, Scatliff

found that another of Brannam’s cocks displayed the same peculiarity, so

purchased him in 1899 for £15, but he also died before the year was out.

Meanwhile Scatliff had, by mating up “Ladybird” with the most likely of

his own cocks, succeeded in producing one or two young cocks with the

desired point. By breeding these with their mother “Ladybird” and their

offspring again with “Ladybird,” Scatliff eventually succeeded in

breeding some turbits, both blacks and duns, with the required

peculiarity fully developed, but not before he had spent a further sum of

£55 on two other cocks, both of which died before they could be mated

with the famous “Ladybird.” However, amid all his misfortunes, Scatliff

informs us that he bought one bird, by name “Amazement,” which did assist

him in fixing his strain. Thus Scatliff spent considerably over £100 in

purchases, and took eight years fixing the peculiarity in question. Had

“Ladybird” been a flower, the peculiarity could probably have been fixed

in one generation by self-fertilisation.

This furnishes an excellent example of the trouble which breeders will

take, and the expense to which they will go in order to produce a desired

result. Nevertheless, it appears to be the fashion for scientific men to

decry the work of the breeder.

Let us now pass on to consider the cases of mutations which are known to

have occurred among animals.

Mutations among Animals

Some instances of great and sudden variation in domesticated animals have

become classical, and been detailed in almost every work on evolution.

These are, firstly, the celebrated hornless Paraguay cattle. This

hornless breed, or rather the ancestor of the breed, arose quite

suddenly.

Many domestic horned breeds of animals, especially sheep and goats, throw

off hornless sports. Were a hornless breed of buffalo found in nature, it

would undoubtedly be ranked a new species, and the Wallaceians would

doubtless exercise much ingenuity in explaining how natural selection had

brought about the gradual disappearance of the horns; and

palæontologists, being baffled in their search for intermediaries between

the hornless species and their horned ancestors, would complain of the

imperfection of the geological record.

It may, perhaps, be argued that this hornless mutation was a direct

result of the unnatural conditions to which the Paraguay cattle were

subjected, it may be asserted that since there are no species of hornless

cattle in nature, such mutations have never occurred under natural

conditions, and hence the Paraguay cattle prove nothing. As a matter of

fact, we know that in nature a great many mutations occur which are not

perpetuated because not beneficial to the species. A hornless individual

in the wild state would stand but little chance in fighting for females

against his horned brethren. We must keep clearly in mind that the theory

of mutation does not seek to abolish natural selection; it merely affords

that force something substantial to work upon.

The second classical example of a leap taken by nature is furnished by

the Franqueiro breed of long-horned cattle in Brazil. These furnish us

with an example of a mutation in the other direction. Then there is the

Niata or bull-dog breed of cattle, which are also South American. These

instances would seem to indicate that cattle are what De Vries would call

“in a mutating state” in that part of the world.

The other classical examples of great and sudden variations are the Ancon

sheep of Massachusetts, the Mauchamp breed of Merino sheep, the tufted

turkeys, and the long-haired race of guinea-pigs.

The “wonder horses,” whose manes and tails grow to an extraordinary

length, so as to trail on the ground, may perhaps be cited as a race

which originated in a sudden mutation. They are all descendants of a

single individual, Linus I., whose mane and tail were respectively

eighteen and twenty-one feet long. But in this case it is important to

note that the parents and grandparents of Linus I. had exceptionally long

hair.

Mutations among Birds

Coming now to birds we find several undoubted examples of mutations, or

new forms which have come suddenly into being.

The black-winged peafowl, whose peculiarities were commented on by

Darwin, afford a striking example of this phenomenon. These birds breed

true when mated together, and are known to have arisen from common

peafowl in no less than nine instances. The cocks have the wings (except

the primary quills), black glossed with blue and green, and have the

thighs black, whereas, in the ordinary peacock, the same part of the wing

is nearly all mottled black and pale buff, and the thighs are drab. The

black-winged hen, on the other hand, is nearly white, but has a black

tail and black speckling on the upper surface of the body, while her

primary quills are cinnamon coloured as in male peafowl, not drab as in

the normal hens. The young are white when hatched, the young cock

gradually assuming the dark colour as he matures.

This mutation, which, in one case quoted by Darwin, increased among a

flock of peafowl until the black-winged supplanted the ordinary kind, is

so distinct in appearance in all stages that it was formerly supposed to

be a true species (_Pavo nigripennis_), of which the wild habitat was

unknown.

The Golden Pheasant (_Chrysolophus pictus_) produces, in domestication,

the dark-throated form (_C. obscurus_), in which the cock has the throat

sooty-black instead of buff, and the scapulars or shoulder feathers black

instead of red. Moreover, the two middle-tail-feathers are barred with

black and brown like the lateral ones, while in the ordinary form they

are spotted with brown on a black ground. The hens have a chocolate-brown

ground-colour instead of yellow-ochre as in the normal type. The chicks

are likewise darker.

The common duck, in domestication, when coloured like the wild mallard,

sometimes produces a form in which the chocolate breast and white collar

of the drake are absent, the pencilled grey of the abdomen reaching up to

the green neck. In this mutation the duck has the head uniformly speckled

black and brown, and lacks the light eye-brow and cheek-stripes found in

the normal duck. Both sexes have the bar on the wing dull black instead

of metallic blue.

The ducklings which ultimately bear this plumage are sooty-black

throughout, not black and yellow like normal ones.

The phenomenon of mutation is not confined to animals in a state of

domestication. The common Little Owl of Europe (_Athene noctua_) has

produced the mutation _A. chiaradiæ_ in the wild state. In this the

irides are dark, instead of yellow as in the normal type, and the plumage

of the back of the wings is longitudinally streaked with white instead of

barred. Several examples of this form were found, along with normal

young, in the nest of one particular pair of little owls in Italy, but

the whole family were foolishly exterminated by local ornithologists.

The reed bunting (_Emberiza schœniclus_) exists in two distinct forms—one

having a much stouter bill than the other (_E. pyrrhuloides_). This

probably is an example of a mutation.

The rare yellow-rumped Finch (_Munia flaviprymna_), of Australia, has

displayed a tendency to change into the allied and far commoner

chestnut-breasted Finch (_M. castaneithorax_) during the lifetime of the

individual (_Avicultural Magazine_, 1907). Conversely, the male of the

common Red-billed Weaver (_Quelea quelea_) of Africa has been found in

its old age to assume the characters of the comparatively rare _Q.

russi_, its black throat becoming pale buff as in that form.

Everyone is familiar with the chequered variety of the common blue-rock

pigeon, in which the wings are regularly mottled with black instead of

being barred. This form sometimes occurs among wild birds, so that it has

been described as a distinct species. It is important to note that there

are red, dun, and silver chequers as well as blue ones.

[Illustration: YELLOW-RUMPED AND CHESTNUT-BREASTED FINCHES, WITH

TRANSITIONAL SPECIMENS]

A well-marked mutation which appears regularly in nature is the

red-headed variety of the beautiful Gouldian Finch (_Pöephila mirabilis_)

of North Australia. Normally the head of the cock is black, but in about

ten per cent. of the individuals the cock has a crimson head, while that

of the hen is dull crimson and black.

Mutations which occur with such regularity are certainly rare. On the

other hand, there are certain mutations which we may expect to see appear

in any species of plant or animal.

Albinistic forms are a case in point, and less frequently we see white

varieties which are not pure albinos, because the eye retains some at

least of the normal pigment. As examples, we may cite white dogs, cats,

fowls, horses, ducks, geese, and Java sparrows among domesticated

animals, and the white forms of the Amazonian dolphin and of the giant

Petrel of the South seas (_Ossifraga gigantea_) among wild creatures.

In a white mutation the eye may lose all its pigment, and then we have a

true albino. Such forms on account of their imperfect vision cannot

survive in a state of nature, hence no wild pink-eyed species are known.

Or the eye may display a partial loss of pigment, as, for example, in the

white domestic forms of the common goose, the Chinese goose, and the

Muscovy duck. Finn saw a case in which the eyes of a pink-eyed rabbit

changed after death into this type of eye—that is, with the pupil black

and the iris blue. It is to be observed that this kind of eye sometimes

occurs in coloured horses, rabbits, and dogs. Finally, we have white

mutations in which the eye loses none of the pigment. These are abundant

in nature, and probably most of the white species of birds—as, for

example, some egrets, swans, etc.—arose in this way.[4] Pure white

species are comparatively uncommon in nature, because, except in

snow-clad regions, white creatures are easily seen by their adversaries.

Most white birds are of considerable size, and well able to look after

themselves.

Similarly black mutations occur frequently among animals, both under

domestication and in a state of nature. All are familiar with black dogs,

cats, horses, fowls, ducks, pigeons. Black mutations, however, do not

occur nearly so frequently as white ones. So far as we are aware no black

mutation has been recorded among canaries, geese, guinea-fowl, ferrets,

Java sparrows or doves, all of which produce white mutations.

On the other hand, in the wild state black species occur more frequently

than normal-eyed white forms. This is probably because such creatures are

less conspicuous than white ones. As examples of black mutations which

occur in nature, we may cite black leopards, water rats, squirrels,

foxes, barking deer (_Cervulus muntjac_), hawk-eagles, harriers, peppered

moth (_Amphidasys betularia_), etc.

That many black species have arisen as sudden mutations from

lighter-coloured animals seems tolerably certain from the facts that in

Malacca the black leopard forms a local race; that some of the Gibbon

apes are as often black as light coloured; that the American black bear

is sometimes brown, while the other bears, when not brown, are almost

invariably black.

Color Mutations

Not uncommon, although rarer than black or melanistic forms, are reddish

or chestnut varieties. These occur both among tame and wild animals.

Among domesticated creatures, sandy cats, “red” pigeons, buff fowls,

chestnut horses, red guinea pigs afford examples of this mutation. Among

wild animals many of the species of squirrel, not naturally red, produce

red mutations; and some of the grey owls—as, for example, the Indian race

of the Scops (_Scops giu_)—throw off a red or chestnut form. As everyone

knows, some species are normally red.

Green or olive species not unfrequently throw off yellow mutations. As

examples of these we may cite yellow canaries, yellow budgerigars

(_Melopsittacus undulatus_), goldfish, golden tench, and the golden form

of the common carp among captive animals; and among animals in a state of

nature, yellow forms have been recorded of the rose-ringed Paroquet

(_Palæornis torquatus_), the green woodpecker, the pike, and the eel.

These lutinistic forms usually have normally coloured eyes. Sometimes,

but only very rarely, these yellow forms throw off white sports—as, for

example, the “silver” form of the goldfish. Finn has seen a white variety

of the common carp. White canaries are excessively rare, while white

budgerigars are unknown.

It is worthy of note that entirely yellow species of birds and fish are

unknown. We would suggest that the explanation of this is that yellowness

is correlated with some physical characteristic unfavourable to an

organism exposed to the struggle for existence; hence individuals which

are yellow are not permitted to survive. In some species of moths

individuals occur in which the parts normally red are yellow. According

to Bateson, a chalk pit at Madingly, near Cambridge, has long been known

to collectors as a habitat of a yellow-marked form of the six-spot Burnet

Moth (_Zygæna filipendulæ_). These lutinistic forms are not confined to

one genus of Butterflies. Moreover, in the Pin-tailed Nonpareil Finch

(_Eythrura prasina_) of the Eastern Archipelago the red tail and other

red parts of the plumage are not infrequently replaced by yellow in wild

individuals of either sex and of any age. In the blue-fronted Amazon

parrot (_Chrysotis æstiva_)—a most variable bird—the normally red edge of

the pinion is sometimes yellow. Bateson, in his _Materials for the Study

of Variation_, gives other examples of this kind of variation.

Mutations among Invertebrates

As further instances of mutations among animals which have been observed

in nature, we may mention the _valezina_ form of the female of the

Silver-washed Fritillary Butterfly (_Argynnis paphia_) and the _helice_

form of the female Clouded-yellow Butterfly (_Colias edusa_).

The common jelly-fish is an organism which frequently throws off sports,

and some zoologists are of opinion that the medusoid _Pseudoclytia

pentata_ arose by a discontinuous variation from _Epenthesis folleata_ or

a closely allied form. Thomson discusses this particular case at some

length on pages 87-89 of his _Heredity_, and gives it as his opinion that

the evidence in favour of this latter having arisen as a mutation is

“exceedingly strong.”

Mutating Species

It is our belief that many species of birds which occur in nature have

been derived from other species which still exist, but as no one has ever

seen the mutation take place, we cannot furnish any proof thereof. We

merely rely on the fact that the species in question differ so slightly

from one another that there seems every likelihood that they have

suddenly arisen and managed to establish themselves alongside of the

parent species.

The Curassows, _Crax grayi_, _C. hecki_, each of which is only known by a

very few specimens, appear to be mutations of the female of the globose

Curassow, _Crax globicera_. The fact that when a female _hecki_ bred in

the London Zoological Gardens with a male _globicera_, the solitary young

one which lived to grow up was a pure _globicera_, renders the assumption

almost certain.

The Chamba Monaul (_Lophophorus chambanus_) seems to be a mutation of the

male of the common Monaul or Impeyan Pheasant (_Lophophorus impeyanus_),

the common species of the Himalayas.

The Three-coloured Mannikin (_Munia malacca_) of South India is probably

simply a white-bellied form of the widely-ranging Black-headed Mannikin

(_M. atricapilla_), which has the abdomen chestnut like the back.

Intermediate wild-caught forms have been recorded.

The African Cordon-bleu (_Estrelda phœnicotis_) and Blue-bellied Waxbill

(_E. cyanogastra_) would also seem to be mutations, as almost the only

difference between them lies in the fact that the male of the former has

a crimson cheek-patch, which is wanting in the latter.

The Ringed Finch (_Stictoptera annulosa_) of Java, and Bicheno’s Finch

(_S. bichenovii_) of Australia, only differ in the former having the rump

black, while in the latter it is white, and this difference appears to be

of the nature of a mutation.

So, it might be urged, is the pure white breast of the male Upland Goose

(_Chloëphaga magellanica_), which part, in the very similar _C. dispar_,

is barred as in the females, the latter form being probably the ancestor.

The differences between the silver-grey-necked Crowned Crane of the Cape

(_Balearica chrysopelargus_) and the dark-necked species of West Africa

(_B. regulorum_) seem also to be not more than could be accounted for by

mutation.

Peculiar forms, such as a rabbit with a convoluted brain or a mouse with

a peculiar pattern of molar teeth, have been come upon by anatomists.

The above-cited mutations are all very considerable ones, and we do not

profess to have mentioned a tenth part of those which have actually been

recorded.

We trust that we have collected and set forth sufficient evidence to show

that the phenomenon of discontinuous variation is a very general one, and

this would seem to tell against the hypothesis of De Vries that species

pass through alternate periods of comparative stability and periods when

swarms of mutations appear. We think it more probable that all species

throw off at greater or less intervals discontinuous variations, and that

it is upon these that natural selection acts.

We further hope that we have succeeded in making clear what we believe to

be the very sharp distinction between continuous and discontinuous

variations, even when the latter are inconsiderable, as frequently

happens.

Somatic and Germinal Variations

Before leaving the subject of variation it is necessary to notice the

distinction, which Weismann was the first to emphasise, between somatic

and germinal variations.

Every adult organism must be regarded as the result of two sets of

forces; inherited tendencies or internal forces, and the action of

environment or external forces. The differences which the various members

of a family show are due in part to the initial differences in the

germinal material of which they are composed, and in part to the

differences of their environment. The former differences are the result

of what we may call germinal variations, and the latter the result of

somatic variations. It is scarcely ever possible to say of any particular

variation that it is a germinal or a somatic one, because even before

birth a developing organism has been subjected to environmental

influences. One of a litter may have received more nourishment than the

others. Nevertheless, any marked variation which appears at birth is

probably largely germinal. According to Weismann and the majority of

zoologists, there is a fundamental difference between these germinal and

somatic variations, in that the former tend to be inherited, while the

latter are never inherited. Weismann believes that very early in the

formation of the embryo the cells which will form the generative organs

of the developing organism are separated off from those cells which will

go to build up the body, and become as much isolated from them as if they

were contained in a hermetically-sealed flask, so that they remain

totally unaffected by any changes which the environment effects in the

somatic cells. Therefore, says Weismann, acquired characters cannot be

inherited.

While the majority of zoologists believe that acquired characters are not

inherited, probably not many will go so far as Weismann and declare that

the environment cannot exercise _any_ effect whatever on the germ cells.

Somatic Variations

Even though acquired characters or variations are not inherited, it does

not follow that they do not play an important part in evolution. Acquired

variations are the result of the way in which an organism reacts to its

environment. If an organism is unable to react to its environment it must

inevitably perish. If it is able to react, it matters not, so far as the

chances of survival of the organism are concerned, whether the adaptation

is the result of a congenital variation or a somatic one. This will be

rendered clear by a hypothetical example. Let us suppose that a certain

mammal is forced, owing to the intensity of the struggle for existence,

to migrate into the Arctic regions. Let us further suppose that this

organism is preyed upon by some creature that hunts by sight rather than

by scent. Let us yet further imagine that this predacious species is

swifter than our animal, on which it preys. It is obvious that, other

things being equal, the more closely the creature preyed upon assimilates

to its surroundings the more likely is it to escape the observation of

its foes, and so to survive and give birth to offspring. Now suppose that

the glare from the snow-covered ground bleaches its coat. This whitening

of the fur is a somatic variation, one which is induced by the

environment. Such an animal will be as difficult to see, if the bleaching

is such as to render it snow-white, as if its whiteness were due to a

germinal variation. Thus, as regards its chances of survival, it matters

not whether its whiteness be the result of germinal or somatic variation.

But if the whiteness is due to a somatic variation, its offspring will

show no tendency to inherit the variation; they will have in turn to

undergo the bleaching process. If, on the other hand, the whiteness is

due to a germinal variation, the offspring will tend to inherit this

peculiarity and to be born white. In such a case, it is unlikely that the

fur of an organism which is naturally coloured will be completely

bleached by the snow, and, even if it be, the bleaching process will take

time, meanwhile the creature will be comparatively conspicuous. So that

those which are naturally whiter than the average, that is to say, those

in which the tendency to whiteness appears as a germinal variation, will

be less conspicuous than those which tend to be the ordinary colour. Thus

the former will enjoy a better chance of survival, and will be likely to

transmit their whiteness to their offspring in so far as it is due to a

germinal or congenital variation.

Thus, although none of the whiteness due to somatic variations is

transmitted to the offspring, such variations are of considerable

importance to the species, as they enable it to survive and allow time

for the germinal variations in the required direction to appear.

That this case need not be purely hypothetical is shown by the fact that

dun domestic pigeons, which are of an earthy-brown colour when fresh

moulted, soon fade in the sun to a dull creamy hue. Thus a coloration

adapted to an ordinary soil could soon be suited to a desert environment.

The ruddy sheldrake also, normally a bright chestnut-coloured bird, and

one that haunts exposed sunny places, in many cases fades very much,

becoming almost straw-coloured.

Many variations which organisms display are of a mixed kind, being in

part the result of inner forces and in part due to the action of the

environment. In so far as they are due to this latter they do not appear

to be inherited.

Thus, although we cannot say of many variations whether they are

germinal, or somatic, or of a mixed kind, it is of great importance to

keep continually in mind the fundamental differences between the two

kinds.

Some somatic variations are due to the direct action of the environment;

they are merely the expression of the manner in which an organism

responds to external stimuli.

What is the cause of germinal variations? This is a question to which we

are not yet in a position to give a satisfactory answer.

The attempt to explain their origin plunges us into the realm of theory.

This doubtless is a realm full of fascination, but it is an unexplored

region of extreme darkness, in which, we believe, it is scarcely possible

to take the right road until more of the light of fact has been shed upon

it.

In the chapter dealing with inheritance we shall indicate the lines along

which it is likely that future progress will be made.