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Lecture 13. Originative Factors in Evolution: Variation.

§ 1. The Central Problem of Ætiology is the Origin of Heritable Variations. § 2. Variations Distinguished from Modifications. § 3. Discontinuous Variations (or Mutations) and Continuous Variations (or Fluctuations) § 4. Problem of the Origin of Variations. § 5. Correlation of Variations. § 6. Theory of Temporal Variations. § 7. Evidences of Definiteness in Variability. § 8. Germ-cells on Implicit Organisms.

§ 1. The Central Problem of Ætiology Is the Origin of Heritable Variations.

WHILE the general idea of evolution is accepted by practically all living naturalists, there is great uncertainty in regard to the factors that have been operative in the process. The uncertainty is partly due to the difficulty of arguing from a meagre experience of the present to a past of many millions of years, and partly to the fact that scientific ætiology is still very young, for it may almost be said to date from Darwin's Origin of Species (1859).

There are two main problems of evolution. The first asks how we are to account for the continual emergence of new things, of changes or variations which make an organism appreciably different from its parents or from the rest of its kin. The second asks what directive factors operate on the variations which arise, determining their elimination or their persistence as the case may be, and working, it may be, towards the familiar but puzzling result—the existence of distinct and relatively well-adapted species. The first question has to do with primary or originative factors; the second has to do with secondary or directive factors. It may well be, however, that the discontinuity of species depends more on originative than on directive factors.

A good many years ago there was born in a normal North of Scotland family a child who grew up to be a wise and well-proportioned dwarf. He married and had children—a certain number of whom were dwarfs. The peculiarity re-appeared in grandchildren and great-grandchildren, and one of the fourth generation was recently at the head of a successful business—a wise and well-proportioned dwarf. The question before us, discussible if not answerable, is, What conditioned the dwarf? This is the fundamental problem of the origin of the distinctively new. Whether it be a clever dwarf, a mathematical genius, a 10-foot tailed cock, a copper-beech, a Greater Celandine with laciniate leaves, the general problem is the same, the old problem of new departures. What are the originative factors in organic evolution?

§ 2. Variations Distinguished from Modifications.

A problem so difficult demands cautious handling. The first question is as to the nature of the novelties that actually occur; and the sound procedure is to take stock of all observed peculiarities or differences marking off individual organisms of the same kind. These “observed differences” must be measured and registered without theory or prejudice. We compare the colour of the trout we catch from different streams, the various numerical relations of radial canals and sense-organs in a thousand jellyfishes of the same species, the plumage in a score of ruffs, the number of vertebræ in a hundred herrings, and so on. We register these observed differences.

It soon becomes plain, however, that analysis of our data is necessary, if we are to avoid fallacy. We must try to sift out peculiarities which are associated with age and with sex, or are directly due to peculiarities of nurture. It is obvious that immature herrings must be compared with immature, and that we must not mix up the ruffs and the reeves, drones and worker-bees. More difficult, however, is it to separate off those peculiarities which can be experimentally shown to be individually acquired modifications, directly due to peculiarities in nurture (whether nutritional, environmental, or functional). Many crabs are profoundly changed by being parasitised by Sacculina and related forms, and a conclusion as to variability in crabs is vitiated by mixing up the parasitised with the normal. An organism dwarfed by lack of food or lack of space for exercise, such as the fresh-water snails studied by Semper and Dc Varigny, is in a different category from a normal dwarf appearing in a family with no dwarfs in its recent lineage. The much cut-up leaves of the fresh-water buttercup in the swiftly flowing water, one of the examples Lamarck gave of the direct results of environmental influence, are not to be placed alongside of the laciniate leaves of a variety of the Greater Celandine (Chelidonium majus) which cropped up without warning in 1590 in an apothecary's garden in Heidelberg, and has been breeding true ever since.

Darwin called these directly induced, exogenous modifications “definite variations”—not a fortunate term; they are currently and unhappily called “individually acquired characters”; they are best called “somatic modifications”. They may be defined as individual bodily changes directly due to peculiarities in environment, nutrition, and function, which transcend the limits of organic elasticity and persist after the inducing causes have ceased to operate. As there is not at present any convincing proof of the transmissibility of these somatic modifications, either as such, or in any representative degree, they must be left out, in the first instance, in our inquiry into the origin of the distinctively new. They may be of great import for the individual, a life-saving veneer, but if they are not transmitted they cannot be of more than indirect importance to the race. It does not follow, however, that a changeful environment may not be an originative factor in evolution.

When we subtract from the total of observed differences those that can be shown to be modifications, when we also eliminate the peculiarities associated with differences of age and sex, the remainder are for the most part (in proportion to the success of our subtraction) what are called variations—inborn not acquired, intrinsic not extrinsic, blastogenic not somatogenic, endogenous not exogenous, arising from the constitution of the germ-cell not impressed from without, expressions not indents. Some of them at least are very transmissible, and it may be said that these constitute the raw materials of evolution.

§ 3. Discontinuous Variations (or Mutations) and Continuous Variations (or Fluctuations).

The next step is to inquire whether all the inborn variations are on the same platform, and here we may go back to Darwin's distinction between (a) “single variations” and (b) “individual variations”, though the terms are not felicitous. (a) By “single variations” Darwin meant sports, abrupt changes, sometimes of notable amount, such as that which gave rise to the copper-beech in the 16th century, or to hornless cattle, or to short-legged sheep, or to Angora rabbits, or to fantail pigeons. They correspond to Galton's “transilient variations”, to Bateson's “discontinuous variations”, to De Vries's “mutations”, and the last name should be kept for them. The contrast, it should be noted, is not so much in the amount as in the kind of change. A white rat does not seem to lack very much to make it a brown rat—the species whence it sprang, but it was in its day a qualitative new departure, and it has bred true. (b) By “individual variations” Darwin meant the minute, ubiquitous peculiarities which distinguish child from parent, brother from brother, cousin from cousin. Though he was much interested in the “single variations” or brusque “sports”, it was in “individual variations” or minute fluctuations that he found most of the raw materials of new species. “The more I work,” he said, “the more I feel convinced it is by the accumulation of such extremely slight variations that new species arise.”

Some authors have tried to identify Darwin's slight individual variations or fluctuations with the somatic modifications already referred to. While this may be sometimes justified in point of fact, Darwin did not regard minute variations as modificational. This is plain from such a sentence as this: “If, as I must think, external conditions produce little direct effect, what the devil determines each particular variation?” Moreover, fluctuations or minute variations often arise among animals whose conditions of life appear to be quite uniform. On the other hand, what Johanssen calls fluctuations in “pure lines” of beans are probably slight modifications due to differences in nurture.

Little is known in regard to the transmissibility of fluctuations or minute variations in the Darwinian sense, but the recent work of Castle (1916), for instance, shows that it is in some cases demonstrable.

It is a curious fact that one of the reasons why Darwin attached little importance to sports or mutations was his belief that they would be swamped in the inter-crossing. In reality they are highly transmissible. When they come they often come to stay unless they are pathological on the one hand, or too superlative, like geniuses, on the other. What is desirable at present is more evidence of the transmissibility of the small fluctuations of germinal origin—a transmissibility which Darwin assumed without question.

To emphasise the contrast between fluctuating or continuous variations, and saltatory or discontinuous mutations, we may quote a couple of vivid sentences from one of Samuel Butler's Essays.

When circumstances are changing, an “organism must act in one or other of these two ways: It must either change slowly and continuously with the surroundings, paying cash for everything, meeting the smallest change with a corresponding modification so far as is found convenient; or it must put off change as long as possible, and then make larger and more sweeping changes”.

“It may be questioned whether what is called a sport is not the organic expression of discontent which has been long felt, but which has not been attended to, nor been met step by step by as much small remedial modification as was found practicable: so that when a change does come it comes by way of revolution. Or, again (only that it comes to much the same thing), a sport may be compared to one of those happy thoughts which sometimes come to us unbidden after we have been thinking for a long time what to do, or how to arrange our ideas, and have yet been unable to arrive at any conclusion.”

To the Dutch botanist De Vries especial credit is due for his recognition of the evolutionary importance of mutations and for his study of their behaviour in inheritance. It is an often told story how he found, in 1886, in a potato-garden near Hilversum, in Holland, a race of the Evening Primrose (Œnothera lamarckiana) in which the mood was all mutation. In spite of Galton's insistence on the reality of transilient variations and Bateson's marshalling of instances of discontinuity, the tendency had grown strong to dogmatise about the continuity of organic change, just as previously about the fixity of species. “Natura non facit saltus,” they said: but De Vries discerned Natura saltatrix in the Evening Primrose of Hilversum, which, by the way, turns out to have been in the 18th century a wild species in North America. Three points may be emphasised. First, that some of the mutants which De Vries's sportive Œnotheras threw off, as an artist might tear sketches from his note-book, were ephemeral failures, while others were viable and bred true, and could not be otherwise described than as species in the making, fingers searching as it were for their appropriate environmental glove. Second, in many cases the mutants were of particular interest because they showed through and through divergences—in leaf and stem and flower—certainly suggestive of some general disturbance of germinal organisation. Just as if the Œnothera was born again! Third, that the creativeness or sportiveness of the Evening Primrose is not restricted to De Vries's particular race of Œnothera lamarckiana. It occurs in other species of Evening Primrose, and also in snapdragon and barley, in strawberry and maize, in pomace-fly and potato-beetle, in rat and in Man himself. Mutations may be induced experimentally, as Professor Tower did with his potato-beetles and as Mme. Henri recently did with the bacillus of anthrax; or they may manifest themselves in wild nature as in the black mutants of Peppered Moth and West Indian Sugar-bird. The result may be a plus or a minus, a dominant or a recessive or neither, pathological or normal. The mutation may occur after crossing or in a pure race; it may show itself potentially before, during, or after fertilisation. In short, there is nothing hard and fast about the origin or nature of mutations: their common features are their brusque appearance, their discontinuity with the parent stock, and their capability of being transmitted intact to a certain proportion of the offspring.

The work of Dr. R. R. Gates on Œnothera lamarckiana is of capital importance. It had been suggested that this species might be a cultivated hybrid, and that its remarkable mutations might be re-combinations of the Mendelian characters of its parents. But it has been shown that Œnothera lamarckiana was in the 18th century at least a wild North American species. Moreover, the brusque phenomena of mutation occur not only in Œ. lamarckiana, but in Œ. biennis, Œ. grandiflora, and Œ. muricata as well.

Of particular interest in many of the mutations of Œ lamarckiana is the fact that they affect several different parts of the plant, including foliage, flowers, and habits. The disturbance produced in the germ-plasm must be of a fundamental character, it has manifold outcrops, as is suggested by the names of the mutants—gigas, lata, nanella, rubricalyx, brevistylis, and so on.

How does Dr. Gates interpret the germinal disturbances which result in somatic mutations? “As regards the ultimate nature of mutations, we are inclined to look upon them as the result of various types of change in the nucleus: (1) morphological changes (a) in number, (b) in shape and size of the chromosomes, or in the arrangement of their substance; (2) chemical or functional changes in (a) whole chromosomes or (b) portions of particular chromosomes, by which a function may be modified or lost; (3) two simultaneous mutations may occur through mismating of the chromosomes in two pairs so that each germ-cell receives both members of one pair; (4) changes in the mysterious karyolymph or gel which forms the groundwork of the nucleus. Such changes may be thought of as alterations in chemical structure or even in polarity, and may also be supposed to extend to the ground substance of the whole cell. But the real nature of all such changes as those last mentioned is at present highly speculative” (1915, p. 303).

§ 4. Problem of the Origin of Variations.

Turning now to the problem of the origin of inborn variations, we may usefully distinguish two levels of difficulty. There are variations and variations. There are some novel-ties that imply just a little more or a little less of some quality,—a slightly longer tail, a slightly denser blackness, a slightly stronger flight-muscle, a slightly weaker eye; some that involve a disappearance of an entire character, such as hair or horns, tail or pigment; some that may be described as obvious re-arrangements of the characters displayed by the ancestry, as we see in a piebald pony or in a hybrid cockatoo. Now it does not seem very difficult to imagine the origin of this kind of quantitative variation. Without pinning our faith as yet to any very detailed view of the material basis of inheritance, we may regard it as certain that the chromosomes play an exceedingly important rôle as vehicles of the heritable qualities. We may compare them to a microscopic pack of cards and we know that they are sometimes visibly different from one another in the same germ-cell, and that there is an extraordinarily elaborate shuffling of the cards before development begins. In the reduction-process involved in the maturation of almost every animal egg-cell, half of the ovum's pack is thrown away, usually in the first polar body, and comes to nothing. In the maturation of the sperm-cell there is also a halving of the pack, but all the reduced units are in this case functional. In fertilisation the two half-packs come together in intimate and orderly union, though without fusion of chromosomes, forming the zygote-nucleus. The opportunities for permutations and combinations of hereditary items, and for the dropping out of one or more altogether, are many and actual. Thus the origin of variations of a quantitative sort does not seem beyond our comprehension, except in the sense that we do not in any way understand the process of cell-division, whether meiotic or reducing division in the maturation of the germ-cells, or the ordinary equational division in other cases.

That this is still only nibbling at the problem is evident when we think of meristic variations (in the number of parts, segments, vertebræ, joints, etc.), which Professor Bateson has usefully distinguished from substantive variations (in the composition of materials). A re-shuffling of the molecular cards within the germ-cell might give rise to a new pigment which was continued in subsequent generations as a definite constituent particle (which we have to credit with great capacity for increase) or as a particular chemical ‘tendency’ of the protoplasm; but how are we to picture the origin and continuance of meristic variations?

A separate consideration may be given to fertilisation as a source of variation,—a view prominent at one stage in the development of Professor Weismann's theories. For a time he was inclined to attach great importance to the mingling (or amphimixis) of two sets of hereditary qualities as a possible source of novelties, but he afterwards attached more importance to the influence that fluctuations in nutrition within the body might have in inducing changes in the germ-plasm or in inducing struggle among the analogous hereditary items. In recent years the Belgian botanist Lotsy has been a thoroughgoing champion of the variational significance of fertilisation and has gone the length of maintaining that all variation is due to crossing. There is ample experimental evidence that novelties may be induced by crossing, and this is not surprising when we remember that two very complex systems, usually of diverse origin, become in fertilisation a unity that goes on in most cases to develop into a harmonious life. On the other hand, Lotsy's attempt to refer all variations to crossing is extreme. This is shown, for instance, by the occasional occurrence of variations in parthenogenetic lineages in which no father intervenes for prolonged periods. Moreover, crossing can be of no avail unless the two sex-cells that combine are different. If they are different it must be by hypothesis because of previous crosses. Thus we simply push the problem back and back to original differences which are left unaccounted for.

The problem before which we are baffled is the origin of the distinctively new, where the novelty is qualitative not quantitative. Some would refuse to admit this distinction, and perhaps they are pedantically right: the distinction is one of common sense. There is many a grade between those who find their fingers indispensable in simple computations, and the calculating boy who can tell us in a few seconds the cube root of 2,498,846,293 yet cannot explain how he knows, but there seems good sense in recognising the latter as a qualitative change. So with the mathematical genius, the musical genius, the artistic genius, and there is not any reason to believe that Man is the only species that produces geniuses. The evidence of their occurrence elsewhere is in the rapidly-growing records of mutations of large amount. There is a mutation-theory, but is there any theory of mutations?

On the dark problem of the origin of the distinctively new some beams of light have been shed. (1) First, there are facts suggesting that deeply saturating environmental influences may act as variational stimuli on the germ-cells and provoke change. Professor MacDougal injected solutions of sugar and compounds of calcium, potassium, and zinc into the developing ovaries of one of the Evening Primroses, and got out of several hundreds of seeds sixteen individuals notably atypical, which bred true to the second and third generation. There were not only losses and augmentations, there were well-marked novelties which maintained their distinctiveness when crossed with the parental strains. It should be noted that what Professor MacDougal injected was not very much out of the way, and might be paralleled by natural changes in the chemical composition of the sap of the plant. Professor Punnett expresses the view of many naturalists when he says: “There is reason to suppose that environmental change leads to abnormal divisions in the ripening germ-cells, and that these abnormal divisions are the starting- point of the new variety” (Article Heredity, Hastings' Encyclopædia of Religion and Ethics).

Pointing in the same direction are the well-known experiments of Professor Tower, who subjected potato-beetles to un-usual conditions of temperature and humidity when the male and female reproductive organs were at a certain stage of development. The results were strangely lacking in uniformity, but some of the offspring showed striking and persistent changes, not only in colour and markings, but also in some details of structure. Professor Tower's work has met with some adverse criticism, but, taken along with similar experiments, it suggests that we must not overlook the possibility of deeply-saturating environmental influences acting as variational stimuli,—affecting not the body of the parent, hut the germ-cells within. Here should be included Weismann's view that fluctuations in. bodily nutrition may prompt the germ-plasm to vary.

(2) Some of the researches of recent years, such as those of Dr. R. Ruggles Gates on Evening Primroses (Œnothera) and of Prof. T. H. Morgan on the Pomace-fly (Drosophila) have focussed attention on the chromosomes. It is a distinct step to know that certain peculiarities of particular mutants are associated with visible alterations in the chromosomes of the fertilised egg-cell. It is very interesting to know that while the fundamental number of chromosomes for the genus Œnothera is 14; this has become 15 in lata and semilata, 21 in semigigas, 28 in gigas, and so on. These are the numbers observed in the fertilised egg-cell and in every element throughout the plant.

In this connection a reference may be made to what obtains in Man. Competent observers have stated that the cells of the male negro have 22 chromosomes, and it is probable that the negress has 24, at least in some cases. Now in the white man and woman the enumerations of Winiwarter and others have usually been 47 and 48. It seems curiously difficult to reach certainty in regard to this simple point, but there is no harm in asking, as Dr. Gates does, whether the white man may not have originated from a black race by a “tetraploid mutation and its consequences”.

The nuclear changes studied in Œnothera in their association with particular mutations are not restricted to changes in the number of chromosomes; they may concern their shape, size, and structure. What has been gained is a demonstration that in some cases the bodily peculiarities of mutants are correlated with visible changes in germinal organisation.

Now one is quite aware that this is just telescoping-down the Proteus of the full-grown organism into the germ-cell phase of its being, and that a recognition of germinal disturbances does not tell us what conditions them. As Professor Bateson has often said, we find ourselves confronted with the oppressive difficulty of cell-division and irregularities in its procedure. Yet there is an enlightening gleam in the proof that somatic mutations are correlated with antecedent germinal disturbances, for we know that abnormal cell-divisions occur in various conditions in Nature, and we have already referred to the opportunities for re-arrangements that occur in the early history and maturation of the germ-cells. Is there any further light?

We must remember that chromosomes are living units in a complex environment, and just as Bacteria sometimes change suddenly in their physiological properties, so chromosomes may vary in their stereochemic architecture or in functional powers. Moreover, it is not fanciful to suppose that these vital units, which have great persistence of ‘individuality’, may exhibit age-changes or periodic reorganisation processes.

Here may be profitably considered the recent work on the Slipper-Animalcule (Paramecium aurelia) by Professor Woodruff and Miss Erdmann. Woodruff has kept a pure line of this Ciliate healthy for over seven years, through more than 4500 generations. As is usual in a pure line all descended from one there was no conjugation. On an average of once a month, however, a remarkable regulatory process occurs, which the authors call endomixis, which secures the indefinite life of the race. Nuclear changes, comparable to those that precede conjugation in normal wild conditions, set in; the old nuclear material, both macro-nuclear and micronuclear, is disintegrated and re-organised. But there is no formation of stationary and migratory micronuclei as there is before conjugation. For conjugation is not going to occur something that takes its place is occurring—endomixis. Now it seems probable that such a periodic re-organisation of nuclear material will afford opportunity for plasmic re-arrangement, and this may imply the origin of variations even within a pure line. Professor Jennings has found in pure lines of non-conjugating Paramecium evidence of variations about the mean. These might be due to re-arrangements effected in endomixis. It is conceivable, as Woodward and Erdmann point out, that “heritable” variations may result from some rare recombinations in endomixis.

This Paramecium is a very complicated organism, as Prof. Clifford Dobell has vividly emphasised, on the non-cellular line of evolution, and we find it in certain conditions exhibiting a monthly re-organisation as part of its life-cycle. Is it not possible that some similar re-organisation may normally occur in Metazoa at the origin of each individual life, and that, if it does, there is no need to look about for any special cause? It is all in the day's work, it is part of the programme of the essentially regulative life-cycle. We may recall, too, that variation occasionally occurs in parthenogenetic or aspermic development, as well as in the ordinary process.

We are not seeking to ‘explain’ variations by verbal inventions. Our argument is quite clear: Certain mutations in organisms are preceded by germinal disturbances, perhaps these germinal disturbances are comparable to endomixis in Paramecium. It is always a step towards understanding to put one obscure process alongside of another which is similar to it and which may be more amenable to experimental treatment. Therefore we suggest that endomixis may be profitably considered along with the problem of the origin of variations.

Another gleam of light may possibly be found in Professor Child's long-continued study of processes of senescence and rejuvenescence,—a study recently presented in its entirety in a remarkable volume Senescence and Rejuvenescence (1915). Professor Child finds that when a fragment of a Planarian regrows a whole, there is a rejuvenescence during the re-constitution; the rate of metabolism is high and the resistance-power is great. The metabolism may be measured by Tashiro's ‘biometer’, an extraordinarily delicate register of the CO2 output, or more indirectly by the degree of susceptibility and resistance to cyanide poisons and the like. Judged by these tests, the regenerating piece of Planarian is younger than it was when it formed part of the parent. It literally renews its youth. Similarly, when a Planarian or a Hydroid multiplies asexually, the separated-off piece shows marked rejuvenescence aa revealed by the two tests named.

Professor Child's thesis is this: As an organism differentiates, it ages, for the accumulation of relatively inactive constituents in the colloidal cytoplasmic substratum necessarily involves a decrease in the metabolic rate; but there are counteractive processes of reduction, removal, and de-differentiation, when the metabolic stream erodes its bed instead of depositing materials. These are marked by acceleration in metabolic rate, and constitute rejuvenescence. “It is certain,” Professor Child says, “that the new individuals which arise by division or budding from other individuals or from experimentally isolated pieces are to some extent physiologically younger than the parent individual from which they arose.”

The idea of a see-saw between processes of senescence and rejuvenescence finds many illustrations among the lower animals, but what of higher levels? Professor Child finds some interesting evidence that the early developmental stages of a number of animal types, before specialisation of cells sets in, are conspicuously young in the physiological sense. The germ-cells themselves are very stable condensations of hereditary items, but in the early development there is a time of re-constitution, of de-differentiation, of relaxation. If there is any soundness in this view, in support of which data are, of course, submitted, we may perhaps recognise another opportunity for variation, namely in the very young embryo, where the alleged rejuvenescence may include possibilities of re-arrangement and, as it were, re-tuning.

§ 5. Correlation of Variations.

The tendency of modern research has been to lay emphasis on the idea of hereditary particulateness, that the characteristics of organisms are made up of elementary units, without intergrades, as sharply separated from one another as the chemical elements. This is the idea of “unit characters”, independently heritable, and independently variable. It is very striking that a trivial feature in. the hands—a reduction of the index and middle finger (in spite of the presence of a little extra triangular bone at their bases), and a consequent projection of the ring finger, should behave as a Mendelian character for at least four generations and be found in fifteen out of thirty-six descendants of the family investigated. (See H. Drinkwater, Journ. Anat. Physiol., L., 1916, pp. 177-186, 14 figs.) There is indirect evidence that particular unit characters are represented by particular particles (factors, determinants, or genes) in the germ-plasm, or perhaps by ultra-microscopic differences of architecture, and the idea works well,—like the atomic theory in chemistry. But it has its limitations and it must not be pressed so hard that we lose sight of the unity of the organism even in the germ-cell phase of its being, and of the fruitful conception of correlated variations. An exaggeration of the idea of particulateness leads to a view which is too mechanical to fit living creatures, as if the organism evolved like a machine perfected piecemeal by the adding on of many little patents independent of each other. A reaction may be seen in the recent book by Prof. T. H. Morgan and others on The Mechanism of Mendelian Inheritance (1915), where it is insisted that the so-called unit character is only the most obvious or most significant product of the postulated ‘factor’, that the effects of a ‘factor’ may be far-reaching and manifold, and that a single character may depend on many ‘factors’ which interact. “Cases of interaction of factors, in which the effect of one factor is altered by the action of another factor, are very numerous” (p. 46). “The expression of a factor-difference may not be limited to one region but may produce a different effect in different regions.”

Many considerations suggest that we should do well to appreciate afresh the idea which Darwin and Sir Ray Lankester have emphasised of the “correlation of variations”, that one change, as we see for instance in disease, may have manifold expression or outcrop in different parts of the body, that the organism may change as a unity in many parts at once. It is not difficult to suppose that a change in the rate of a particular kind of metabolism may reverberate through the body. As Mr. J. T. Cunningham and Professor Dendy have pointed out, an augmentation or a diminution of certain internal secretions or hormones might have multitudinous transforming effects.

§ 6. Theory of Temporal Variations.

Another important idea is that of temporal variations, that is to say alterations in the tempˆ, or rate, or rhythm of metabolic processes, or in the duration of particular phases in the life-cycle. Many changes of great adaptiveness are probably due to a lengthening out of one chapter and the telescoping of another. In the remarkable regulatory influence of the internal secretions in backboned animals we get a hint as to the way in which changes in ‘time’ might be effected.

It is very interesting to compare different life-histories from this point of view. In some, such as May-flies or Ephemerides, the adult life is condensed into a few days or even hours. It may even be lost altogether as in cases of pædogenesis, where there is juvenile reproductivity. On the other hand, when juvenile life is hazardous, it may be, as it were, telescoped down into the egg; thus the young Mound-bird is able to fly on the day on which it is hatched. In other cases, as in the generations of Planarians badly fed, the animal may be born old. Part of the tune may be played very slowly, part very quickly, and another part left out altogether, and a life-history adaptive to particular conditions may be the result of selecting out suitable temporal variations. (See in this connection Mitchell, 1912, and Thomson, 1914.)

We must not think too exclusively of variations in structure; many variations may affect rate and intensity; many may be differences in stability of constitution, in rapidity of reflexes and cerebral, processes, and in the mysterious quality called vigour. Or, penetrating further still, may we not recognise the possibility of a kind of variation which is of more profit than any increase of stature, strength, or speed, than any perfection of armour or weapons, than any subtlety of protective coloration or mimetic resemblance,—a kind of variation that expresses itself in a keener endeavour after well-being, a stronger will to live, and a livelier sense of kinship?

§ 7. Evidences of Definiteness in Variability.

For our interpretation of evolution it is important to recognise the growing body of evidence that variation is a much more definite, much less fortuitous, organic change than was formerly supposed. (A) There are many illustrations of what is called orthogenesis, or progressive variation along a definite line. The palæontologists in particular have very strong convictions as to reality of this orthogenesis, and as to the absence of arrows shot at a venture. (B) Instances are accumulating of the occurrence of mutations or brusque variations, and if these come they often come to stay. The Black Mutant of the Peppered Moth was rare 60 years ago; in many places it has now replaced the originative stock. This lessens the element of the casual in organic evolution. It also lessens the need for over-burdening the rôle of natural selection in sifting out from amid a crowd of random novelties, and as an accumulator of minute increments. (C) But along with this there should be considered the idea, that variations are limited in some measure by what has gone before. At the beginning of each individual life there is the fertilised ovum,—a viable unity. If a variation occur it is not like to grip unless it be congruent with the germinal organisation already established; it must harmonise, just as an addition to a crystal must, but within a wider range. The character of the building that has been erected determines in some measure the nature of an addition to it. The idea of architecture is of course only one aspect; the novelty must be congruent with the previously established reaction system and specific metabolism. Out of the same spring we do not get sweet water and bitter. This is an old but important idea; we find in Aristotle the suggestion that the possible range of the form of an organ is limited to some extent by its existing differentiation. Thus the element of the fortuitous shrinks still further. It is interesting to find that monsters sometimes result from infelicitous crossings, but perhaps a greater interest attaches to the fact that monsters are rare in Nature, not only in survival, but in occurrence.

An illustration of the limiting of changes by pre-existing organisation may be found in a recent paper by Prof. S. J. Hickson (Mem. & Proc. Manchester Lit. & Phil. Soc., LX., 1916, pp. 1-15), in which he notes that meristic variability in important organs is much greater in radially symmetrical forms than in bilaterally symmetrical forms where a balance must be kept. In reference to the Pennatulacea he shows that variable or plastic characters may become less variable or plastic as a transition is made from radial to bilateral symmetry, and points out that increasing rigidity of certain. characters leads in some cases to the differentiation of the discontinuous groups which are recognised as species. What we would suggest is carrying this idea from the fully-formed organism to the germ-cell organism, and considering substantive as well as meristic variations.

§ 8. Germ-cells as Implicit Organisms.

Let us sum up. Germinal disturbances or re-arrangements occur and these may find expression in development as variations or mutations of the organism. The question is, What brings about the re-arrangements?—a question to be asked in the light of the fact that, frequent as variations are, hereditary constancy, or inertia, or persistence of specificity is even more marked. The following suggestions are before us. (1) That germinal disturbances come about in response to subtle environmental stimuli of a novel kind penetrating in from without and affecting cell-division, or the architecture of the chromosomes, or perhaps the “mysterious karyolymph or gel which forms the groundwork of the nucleus”. Along with definable changes in the external environment may be included changes in the somatic fluids which might affect the nutritive or other metabolism of the germ-cells. (2) That in the division of the germ-cells before fertilisation, where there has to be a partition of a complex cytoplasmic and chromosomic cargo between two vessels, losses and augmentations and inequalities may be expected in the transhipment. (3) That in fertilisation, with its intimate and orderly union of paternal and maternal contributions (amphimixis), there may be opportunity for new permutations and combinations, the result normally being a viable unity of dual origin. (4) That there may be growth-changes, or regulative reorganisation processes, or rejuvenescences in the germ-cells in the course of their history; and it is possible that there may be something in Weismann's hypothesis of intra-germinal struggle.

We are thus aware of certain originative factors in evolution, which admit of experimental testing, and we should not lose sight of any of them. Each must be pushed as far as it will go. Recognising this, some will insist that there is no more to be said, but much to be done. We venture to doubt, however, whether this is not making a tyranny of scientific method (which, after all, is very selective and partial) and giving up the right of speculative adventure. As Karl Ernst von Baer, the great Russian embryologist, said: There is observation, but there is also reflection.

Those who have devoted much attention to the occurrence of variation, we think for instance of Darwin and Bateson, have given emphatic expression to their sense of the difficulty of accounting for the origin of the new. The fountain of change, whence are its well-springs? “As to almost all the essential features, whether of cause or mode, by which specific diversity has become what we perceive it to be, we have to confess an ignorance almost total” (Bateson, 1913, p. 248). But we also notice that some of those who have given much of their life to the study of the phenomena of variation occasionally lapse from the stern path of science, and in face of the difficulty of the problem ask themselves if they are allowing enough for the fact that the organism is alive. Thus we would quote from the recent work of Dr. R. R. Gates on The Mutation Factor in Evolution this interesting sentence: “Just as an Alpine climber dangling over a chasm may, by changing his hold, swing himself on to a shelf from which he can make a fresh start in some other direction, so we may think of the organism trying many unconscious experiments in its offspring, some of which are hurled by the gravitational effect of natural selection into the abyss of extinction while others with a more fortunate turn rest on a ledge of safety whence new essays of variability begin.” But Dr. Grates mutationist all too speedily takes the place of Dr. Gates psycho-biologist. After this one exciting glimpse of the organism as climber we are hurried back to the chemical and physical complexity of the protoplasm and its unique irritability and retentiveness. But we are disposed to linger over the idea of the organism as climber, and the organism here means the germ-cell. It is not suggested that the germ-cell is dominated by any purpose of getting to the top of anything, or of circumventing any particular difficulty, but rather that there is inseparable from it a restless experimenting in self-expression, bearing the same relation to the insurgent self-assertiveness of the full-grown creature that the tentatives of dreamland bear to the achievements of open-eyed and deliberate endeavour.

The position we are suggesting is that the larger mutations, the big novelties, are expressions of the whole organism in its germ-cell phase of being, comparable to experiments in practical life, solutions of problems in intellectual life, or creations in artistic life. These are accomplished, every one knows, by molecular activities in the brain and body, but they are not intelligibly thought of unless we conceive of the organism as a psycho-physical individuality, a mind-body, or body-mind, as we will. Similarly it may be that our conception of germinal variability is falsely abstract unless we recognise that germ-cells are living individualities of great complexity telescoped-down into a one-celled phase of beings, and that they too make essays in self-expression.

Mr. E. S. Russell (1915, p. 430) has suggested that non-adaptive specific differences which make species discontinuous may be profitably compared to the differences between related organic compounds, and that they may be due to differences in metabolism or stereochemic architecture which cannot be other than discontinuous. But adaptive specific characters, whether of internal or external reference, may be the result in the long run of some “obscurely psychic capacity for active effort”. “The analogies between intelligent and instinctive behaviour on the one hand and the organic processes of active adaptation on the other, as these are expressed in changes of form, are striking and profound.” Mr. Russell goes on to say that behaviour and morphogenesis are probably different manifestations of one and the same fundamental capacity which cannot be formulated adequately without using psychical terms.

If it be said that this is retrograde science to fall back on psychical formulation because of the baffling difficulty of physiological formulation, and that it is a reversion to the mediaeval solution of the problem of digestion and the like by calling in the aid of Archegæus and other indwelling spirits. But it may be answered first, that in giving an account of our own behaviour it is not a hypothesis to regard psychical factors as verœ causœ, that somehow or other the psychical factors we are aware of were implicit in the germ-cell whence we sprang, and that therefore it is not a reversion to mediæval physiology to keep our mind open to the possibility that the origin of the profounder and more vital variations may not be statable without a recognition of the implicit organism of the germ-cell as at once psychical and metabolic.

Perhaps we mislead ourselves by repeating too often the elementary commonplace that the Metazoon begins its life as a single cell. It is true enough in a way, but certainly not the whole truth. It is no commonplace cell, the gamete. It is an implicit organism and within it, in some manner that we cannot begin to image, though we crowd it with factors and genes (the modern successors of Darwin's gemmules and Weismann*'s determinants), there lies a complex inheritance, unified afresh at the start of each new generation. If an Amœba has a behaviour, as Professor Jennings seems to have proved, may not the much more richly-endowed germ-cell of a fruit-fly be allowed the capacity of putting its house in order? If the Foraminifer Technitella thompsoni picksand chooses the materials of its encasement and builds this with what looks like a dawning art, may not the ovum of an Evening Primrose be allowed some freedom of internal architecture? Germ-cells are not corpuscles of undifferentiated protoplasm. They are individualities that live and multiply, that struggle and combine. They are repositories of multiplicate inheritances borne by strangely persistent smaller living units, the chromosomes, which adjust themselves in the most momentous of organic compromises. Is it fanciful to suppose that these gametes, neither simple cells nor portmanteaus of hereditary factors, but unified individualities, experiment internally, not fortuitously but artistically, not at random nor yet inexorably, not purposefully but perhaps purposively, and that they are body-minds or mind-bodies telescoped down?

In certain moods one feels inclined to agree with those who say to-day what Darwin said more than fifty years ago that our ignorance of variation is profound, and who urge the appropriateness of silence. Yet perhaps it is more wholesome for thinkers to have an exposure of the vast uncertainty that surrounds this central problem of biology than to be led astray by those who confidently declare that organic evolution can be mechanically re-described. If the redescription is difficult and still impossible when we use full-blooded biological categories, how must it be if matter-and-motion categories are supposed to be the only legitimate ones?

SUMMARY.

The central and most difficult problem of ætiology concerns the origin of the new, that is, of those variations or mutations that form the raw materials of progress or the reverse.

From an unbiassed registration of all observed differences between the members of the same species there have to be subtracted all the peculiarities that can be reasonably interpreted as associated with age and sex, or as individually-acquired somatic modifications directly due to peculiarities of nurture, whether environmental, nutritional, or functional. As there is no convincing evidence at present that these extrinsic somatic modifications can be transmitted as such, or in any representative degree, they cannot be included, in the first instance at least, among the raw materials of racial evolution. These are discerned when the modifications in question and the peculiarities associated with age and sex are subtracted from the total of observed differences. For this subtraction brings into view the true variations or mutations—inborn not acquired, blastogenic not somatogenic, endogenous not exogenous, expressions or outcomes not indents or imprints. Some at least are very transmissible, and these furnish the raw materials of evolution.

Among inborn variations it is useful to distinguish between mutations (Galton's “transilient variations”, Bateson's “discontinuous variations”) and small fluctuating variations. The former arise brusquely, with a measure of perfectness from the first, without intergrades, and are markedly transmissible. The latter are of the nature of “a little more or a little less”, they show intergrades; their transmissibility has not been much studied, but it has been proved in a few cases.

It is also useful to distinguish quantitative variations from definite novelties. The reduction or exaggeration of a quality, the dropping-out of a character altogether, a re-arranged pattern of hereditary items, may be called quantitative, and may be explained as due to permutations and combinations of the determinants or factors of hereditary characters. For such shufflings of the cards ample opportunities are afforded in the course of the maturation of the germ-cells.

Another possibility is afforded at the beginning of each individual life, where, in the great majority of cases, two very complex systems of dual origin become a new unity which normally develops into a harmonious organism. Some modern evolutionists attach great importance to crossing as a cause of variations.

But the greater difficulty is with the origin of the distinctively new, of what may be called qualitative variations or mutations, (a) It may be that deeply-saturating environmental influences act as variational stimuli on the germ-cells, provoking change, (b) Definite changes in the nuclear bodies or chromosomes of the germ-cell have been proved to be associated with particular mutations in the full-grown organism, and, in addition to the opportunities for chromosomes change afforded in the history of the germ-cells—before, during, and after fertilisation—it is possible that chromosomes, which are living units, may change suddenly like Bacteria, or may undergo age-changes, or may exhibit periodic re-organisation like slipper-animalcules, or rejuvenescence-changes like those occurring in some cases of regeneration and asexual multiplication.

The tendency of modern research is to emphasise the idea of particulateness, for it looks as if the characteristics of organisms were often made up of elementary units, without intergrades, as sharply separated from one another as the chemical elements. But we must not lose sight of the unity of the organism, even in the one-cell phase of its being, and of the correlation of variations. A change in some particular kind of metabolism may reverberate through the whole body.

Another important idea is that of temporal variations, that is to say, alterations in the ‘time’ or rate or rhythm of metabolic processes, or in the duration of particular phases in the life-cycle. Many changes of great adaptiveness are probably due to the lengthening out of one chapter and the telescoping of another. In the influence of internal secretions in backboned animals there is a known method of effecting these changes in ‘time’

Of great importance for our interpretation of evolution is the growing body of evidence that variation is often a much more definite organic change than was formerly supposed. There are many illustrations of progressive variation along a definite line,—orthogenesis. Instances of mutations are accumulating, and if mutations come they usually come to stay. This also lessens the element of the casual and the need for over-burdening Natural Selection with the task of sifting from amid a crowd, and of accumulating minute increments. Furthermore, variations occurring in a unified individuality are not likely to be stable unless they are congruent with the organisation already established. Thus there seems little warrant for talking about evolution as a “chapter of accidents”

It may well be that our conception of variability is fallaciously abstract unless we recognise that germ-cells are living organisms of great complexity telescoped down into a one-cell phase of being, and that they make essays in self-expression which we call variations. These blind experiments of the germ-cells are submitted to the developing and adult organism to be tested in actuality.