Chapter VI · ORGANIC EVOLUTION. (continued 2)

These facts are of no little importance. They tend to minimize, for some characters at least, the effects of intercrossing. The variations which present this trait may be likened to stable organic compounds, which may be inherited or not inherited, but which cannot be watered down by admixture and intercrossing. It is well known[DX] that, in 1791, a ram-lamb was born in Massachusetts, with short, crooked legs and a long back, like a turn-spit dog. From this one lamb[DY] the otter, or ancon, breed was raised. When sheep of this breed were crossed with other breeds, the lambs, with rare exceptions, perfectly resembled one parent or the other. Of twin lambs, even, one has been found to resemble one parent, and the second the other. All that the breeder has to do is to eliminate those which do not possess the required character. And very rarely do the lambs of ancon parents fail to be true-bred.

Now, it can scarcely fail that such sports occur in nature. And if they are stable compounds, they will not be readily swamped by intercrossing. It only requires some further isolation to convert the sporting individuals into a distinct and separate variety. Now, Darwin tells us that the ancons have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. Here, then, we have preferential mating as the further isolating factor. I feel disposed, therefore, to agree with Mr. Galton when he says,[DZ] "The theory of natural selection might dispense with a restriction for which it is difficult to see either the need or the justification, namely, that the course of evolution always proceeds by steps that are severally minute, and that become effective only through accumulation. That the steps may be small, and that they must be small, are very different views; it is only to the latter that I object, and only when the indefinite word 'small' is used in the sense of 'barely discernible,' or as small as compared with such large sports as are known to have been the origins of new races."

Connected, perhaps, with the phenomena we have just been considering is that of prepotency.[EA] It is found that, when two individuals of the same race or of different races are crossed, one has a preponderant influence in determining the character of the offspring. Thus the famous bull Favourite is believed to have had a prepotent influence on the short-horn race; and the improved short-horns possess great power in impressing their likeness on other breeds. The phenomena are in some respects curiously variable. In fowls, silkiness of feathers seems to be at once bred out by intercrossing between silk-fowl and any other breed. But in the silky variety of the fan-tail pigeon this character seems prepotent; for, when the variety is crossed with any other small-sized race, the silkiness is invariably transmitted. One may fairly suppose that prepotent characters have unusual stability; but to what causes this stability is due we are at present ignorant.

Lastly, we have to consider the phenomenon of latency. This is the lying hid of characters and their subsequent emergence. We may distinguish three forms of latency.

1. Where characters lie hid till a certain period of life, and then normally emerge.

2. Where the characters normally lie hid throughout life, but are, under certain circumstances, abnormally developed.

3. Where the characters lie hid throughout life, but appear in the offspring or (sometimes distant) descendants.

Latency is often closely connected with correlated variations. Secondary sexual characters, for example, are correlated with the functional maturity or activity of the reproductive organs. They therefore lie hid until these organs are mature and ready for activity. When they are restricted to the male, they normally remain latent throughout the life of the female, but reappear in her male offspring. Under abnormal conditions, such as the removal of the essentially male organs, the secondary sexual characters correlated with them do not appear, or appear in a lessened and modified form. The males may even, under such circumstances, acquire female characters. Thus capons take to sitting, and will bring up young chickens. Conversely, females which have lost their ovaries through disease or from other causes sometimes acquire secondary sexual characters proper to the male. Characters thus normally latent abnormally emerge. Mr. Bland Sutton[EB] gives a case of a hen golden pheasant which "presented the resplendent dress of the cock, but her plumage was not quite so brilliant; she had no spurs, and the iris was not encircled by the ring of white so conspicuous in the male." Her ovary was no larger than a split pea.

A curious instance of latent characters correlated with sex is seen in hive bees. The worker bee differs from the female in the rudimentary condition of the sexual organs, in size and form, and in the higher development of the sense-organs. But it is well known that, if a very young worker grub be fed on "royal jelly," she will develop into a perfect queen. Not only are the sexual organs stimulated to increased growth and functional activity, but the correlated size and condition of the sense-organs are likewise acquired. The characters of queen and worker are latent in the grub. According to the nature of the food it receives, the one set of characters or the other emerges. Professor Yung's tadpoles and Mrs. Treat's butterflies (ante, p. 59) afford similar instances.

We come now to those cases of latency in which this obvious correlation does not occur. They afford examples of reversion to more or less remote ancestral characters. In some cases the cause of such reversion—such unexpected emergence of characters, which have remained latent through several, perhaps many, generations—is quite unknown. In others, at any rate among domesticated animals, the determining condition of such reversion is the crossing of distinct breeds.

Darwin gives[EC] an instance of reversion, on the authority of Mr. R. Walker. He bought a black bull, the son of a black cow with white legs, white belly, and part of the tail white; and in 1870 a calf, the gr-gr-gr-gr-grandchild of this cow, was born, coloured in the same very peculiar manner, all the intermediate offspring having been black. In man partial reversions are not infrequent. An additional pair of lumbar ribs is sometimes developed, and in such cases the fan-shaped tendons which are normally connected with the transverse processes of the vertebræ are replaced by functional levator muscles. Since it is probable that the ancestor of man had more than the twelve pairs of ribs that are normally present in the human species, we may, perhaps, fairly regard the supernumerary rib as a reversion. But it may be a new sport on old lines.

The occasional occurrence in Scotland of red grouse with a large amount of white in the winter plumage, especially on the under parts, is justly regarded by Mr. Wallace[ED] as a good example of reversion or latency in wild birds. There can be little doubt that, as he suggests, the Scotch red grouse is derived from a form which, like the wide-ranging willow grouse, has white winter plumage. During the glacial epoch this would be an advantage. "But when the cold passed away, and our islands became permanently separated from the mainland, with a mild and equable climate, and very little snow in winter, the change to white at that season became hurtful, rendering the birds more conspicuous, instead of serving as a means of concealment." The red grouse has lost its white winter dress; but occasional reversions point to the ancestral habit.

That crossing tends to produce reversion is a fact familiar to breeders and fanciers, and one which is emphasized by Darwin. When pigeons are crossed, there is a strong tendency to revert to the slatey-blue tint and black bars of the ancestral rock-pigeon. There is always a tendency in sheep to revert to a black colour, and this tendency is emphasized when different breeds are crossed. The crossing of the several equine species (horse, ass, etc.) "tends in a marked manner to cause stripes to appear on various parts of the body, especially on the legs," and this may be a reversion to the condition of a striped and zebra-like ancestor. Professor Jaeger described a good case with pigs. "He crossed the Japanese, or masked breed, with the common German breed, and the offspring were intermediate in character. He then recrossed one of these mongrels with a pure Japanese, and in the litter thus produced one of the young resembled in all its characters a wild pig; it had a long snout and upright ears, and was striped on the back. It should be borne in mind that the young of the Japanese breed are not striped, and that they have a short muzzle and ears remarkably dependent."[EE] Darwin crossed a black Spanish cock with a white silk hen. One of the offspring almost exactly resembled the Gallus bankiva, the remote ancestor of the parents.

Such cases would seem to show that in our domestic breeds ancestral traits lie latent. The crossing of distinct varieties may either neutralize the variations artificially selected, and thus allow the ancestral characters which have been masked by them to reappear; or they may allow the elements of the ancestral traits, long held apart in separate breeds by domestication, to recombine with the consequent emergence of the normal characters of the wild species. But, in truth, any attempted explanations of the facts are little better than guess-work. There are the facts. And the importance of crossing as a determining condition in domesticated animals should make us cautious in applying reversion, as it occurs in such cases, to wild species which live under more stable conditions where crossing is of rare occurrence.

The Origin of Variations.

The subject of the origin of variations is a difficult one, one concerning which comparatively little is known, and one on which I am not able to throw much light.

Taking a simple animal cell as our starting-point, we have already seen that it performs, in primitive fashion, certain elementary and essential protoplasmic activities, and gives rise to certain products of cell-life. In the metazoa, which are co-ordinated aggregates of animal cells, together with some of their products, there is seen a division of labour and a differentiation of structure among the cells. We see, then, that variation among these related cells has led to differences in size, in form, in transparency, and in function; while the cell-products have been differentiated into those which are of lifelong value, such as bone, cartilage, connective tissue, horn, chitin, etc., together with a variety of colouring matters; those which are of temporary value, such as the digestive secretions, fat, etc.; and those which are valueless or noxious, such as carbonic acid gas and urea, which are excreted as soon as possible. Here are already a number of important and fundamental variations to be accounted for.

Let us notice that, wide as the variations are, they are to a large extent hedged in by physical, chemical, and organic limitations. We have already seen that the size of cells is to a large extent limited, because during growth mass tends to outrun surface; and because, while disruptive changes occur throughout the mass, nutriment and oxygen must be absorbed by the surface. This is a physical limitation. Since the products of cell-life and cell-activity are chemical products, it is clear that they can only be produced under the fixed limitations of chemical combination; and though in organic products these limitations are not so rigid as among inorganic substances, still that there are limitations no chemist is likely to question. The organic limitations are to the varied, but not very numerous, modes of protoplasmic activity.

Probably, even at the threshold of metazoan life, such variations did not affect only individual cells, but rather groups of cells. In other words, the differentiation was at once and primarily a tissue-differentiation. What do we know, however, about the primitive tissue-differentiation of the earliest metazoa? Hardly anything. We may fairly suppose that the first marked difference to appear was that between the outside and the inside. In the formation of an embryo this is the first differentiation we notice. From the beginning of segmentation or, in any case, very early, the outer-layer cells become marked off from the inner-layer cells. The next step was, perhaps, the formation of the mid-layer between the outer and inner. But how further differentiations were effected we really do not know, though we may guess a little. This, perhaps, we may fairly surmise—that fresh differentiations presupposed previous differentiations, and formed the basis of yet further differentiations. Thus calcified cartilage presupposes cartilage, and leads up to the formation of true bone. In all this, however, we are very much in the dark. We can watch, always with fresh wonder, the genesis of tissues in the development of the embryo; but we do not at present know much of the mode of their primitive genesis in the early days of organic evolution: how can we, then, pretend to understand their origins?

If we speculate at all on the matter, we are led to the view that the variations must be primarily due to the differential incidence of mechanical stresses and physical or chemical influences. It may be admitted that this is little more than saying that they are due to some physical cause. Still, this at least may be taken as certain for what it is worth—that the primitive tissue-differentiations are due to physical or chemical influences, direct or indirect, on the protoplasm of the cell. Here is one mode of the origin of variations.

I do not wish to reopen the question whether these variations originate in the germ or in the body. I content myself with indicating the difference, from this standpoint, between the two views. Take, for example, the end-organs of the special senses, which respond explosively to physical influences in ways we shall have to consider more fully in the next chapter. If we hold that variations originating in the body may be transmitted through the germ to the offspring, then we may say that these variations are the direct result of the incidence of the physical or molecular vibrations on the protoplasm. But if we believe, with Professor Weismann, that all variations originate in the germ, then the variations in the end-organs of the special senses, fitting them to be the recipients of special modes of influence, result from physical effects upon the germ of purely fortuitous origin, that is to say, wholly unrelated to the end in view. The rods and cones of the retina are due to purely chance variations, impressed by some chemical or physical causes completely unknown on the germinal protoplasmic substance. Those individuals which did not have these chance variations have been eliminated. It matters not that the rods and cones are believed to have reached their present excellence through many intermediate steps from much simpler beginnings. The fact remains that the origin of all these step-like variations was fortuitous, and not in any way the direct outcome of the physical influences which their products, the rods and cones, have become fitted to receive. I am not at present prepared to accept this theory of the germinal origin of all tissue-variations.

Whether use and disuse are to be regarded as sources of origin of variations is, again, a matter in which there is wide difference of opinion. But if we admit that any variations can take their origin in the body (as distinguished from the germ), then there is no à priori reason for rejecting use and disuse as factors. As such, we are, I think, justified, in the present state of our knowledge, in reckoning them, at all events, provisionally.

It is clear, however, that they are a proximate, not an ultimate, source of origin. I mean that the structures must be there before they can be either strengthened or weakened by use or disuse. They are at most a source of positive or negative variations of existing structures. They cannot be a direct source of origin of superficial variations. Gain or loss of colour; form-variations not correlated with organic variations;—these cannot be directly due to use or disuse. It is in the nervous and muscular systems and the glandular organs that use and disuse are mainly operative. When, however, organs are brought into relation, or fail to be brought into relation, to their appropriate stimuli, we speak of this, too, as use and disuse. We say, for example, that persistent disuse may impair the essential tissues of the recipient end-organs of the special senses, implying that these tissues require to be brought into continued relation to the appropriate stimuli in order that their efficiency be maintained. So, too, we say that the epidermis is thickened by use, meaning that it is brought into relation with certain mechanical stresses. Through correlation, too, the effects of use and disuse may be widespread. Thus increase in the size of a group of muscles may be correlated with increase in the size of the bones to which they are in relation. In fact, so knit together and co-ordinated is the organism into a unity, it is probable that hardly any variation could take place through use or disuse without modifying to some extent the whole organic being.

Once more, let it be clearly remembered that a large and important school of zoologists reject altogether use or disuse as a factor in variation. They believe that those germs are selected through natural selection in which there is an increased tendency to use or disuse of certain organs. In this, however, we are all agreed. The real question is what is the source of origin of this tendency. On the view of germinal origin, we are forced back on unknown physical or chemical influences in no wise related in origin (though, of course, related in result) with the use or disuse to which they give rise.

So far the main distinction between the two biological schools seems to be that the one, placing the origin of variation in the body-tissues, regards the variations as evoked in direct reaction to physical or chemical influences; while the other, placing the origin of variation in the germ, regards the variations as of fortuitous origin.

I do not use the phrase, "of fortuitous origin," as in any sense discrediting the theory. I am not attempting the cheap artifice of damning a view that does not happen to be my own with a phrase or a nickname. And I therefore hasten to point out what variations I do believe to have had a fortuitous origin. The phrase is often misunderstood, and they will serve to explain its meaning.

If the reader will kindly refer to the tables of variations in the bats' wings (Figs. 14-17), he will see that there are a great number of bones which vary in length and vary independently. And if he will also refer to Fig. 18, in which seven species of bats are compared, he will see that the differences arise from the increased length of one set of bones in one species and another set of bones in another species. Now, let us suppose that the long, swallow-like wing of the noctule, a high flyer with rapid wing-strokes, that catches insects in full flight, and the broad wings of the horse-shoe, a low flyer, flapping slowly, and, at any rate, sometimes catching insects on the ground, and covering them with its wings as with a net; let us suppose, I say, that to each species its special form of wing is an advantage. Among thousands of independent variations in the lengths of the bones there would be occasional combinations of variations, giving either increased length or increased breadth to the wing. In the noctule, the former would tend to be selected; in the horse-shoe, the latter. Thus the wing of the noctule would be lengthened, and that of the horse-shoe broadened, through the selection of fortuitous combinations of variations which chanced to be favourable. Now, each individual bone-variation is, we believe, due to some special cause; but the fortunate combination is fortuitous, due to what we term "mere chance."

Darwin believed that chance, in this sense, played a very important part in the origin of those favourable variations for which, as he said, natural selection is constantly and unceasingly on the watch. And there can be little question that Darwin was right.

We must now consider very briefly some of the proximate causes of variations. In most of these cases we cannot hope to unravel the nexus of causation. When a plexus of environing circumstances acts upon a highly organized living animal, the most we can do in the present state of knowledge is to note—we cannot hope to explain—the effects produced.

All readers of Darwin's works know well how insistent he was that the nature of the organism is more important than the nature of the environing conditions. "The organization or constitution of the being which is acted on," he says,[EF] "is generally a much more important element than the nature of the changed conditions in determining the nature of the variation." And, again,[EG] "We are thus driven to conclude that in most cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays when a mass of combustible matter bursts into flame—the nature of the flame depending on the combustible matter, and not on the spark."

Recent investigations have certainly not lessened the force of Darwin's contention. From which there follows the corollary that the vital condition of the organism is a fact of importance. Darwin was led to believe that among domesticated animals and plants good nutritive conditions were favourable to variation. "Of all the causes which induce variability," he says,[EH] "excess of food, whether or not changed in nature, is probably the most powerful." Darwin also held that the male is more variable than the female—a view that has been especially emphasized by Professor W. K. Brooks. Mr. Wallace, as we have already seen, regards the secondary sexual characters of male birds as the direct outcome of superabundant health and vigour. "There is," he says,[EI] "in the adult male a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury." And Messrs. Geddes and Thomson contend[EJ] that "brilliancy of colour, exuberance of hair and feathers, activity of scent-glands, and even the development of weapons, are in origin and development outcrops of a male as opposed to a female constitution."

There is, I think, much truth in these several views thus brought into apposition. Vigour and vitality, predominant activity and the consequent disruptive changes, with their abundant by-products utilized in luxuriant outgrowths and brilliant colours, are probably important sources of variation. They afford the material for natural selection and sexual selection to deal with. These guide the variations in specific directions. For I am not prepared to press the theory of organic combination so far as to believe that this alone has served to give definiteness to the specific distinctions between secondary sexual characters, though it may have been to some extent a co-operating factor. This, however, is a question apart from that of origin. Superabundant vigour may well, I think, have been a source of origin, not only of secondary sexual characters, but of many other forms of variation.

And while these forms of variation may be the special prerogative of the male, we may perhaps see, in superabundant female vigour, a not less important source of developmental and embryonic variations in the offspring. The characteristic selfishness of the male applies his surplus vitality to the adornment of his own person; the characteristic self-sacrifice of the mother applies her surplus vitality to the good of her child. Here we may have the source and origin of those variations in the direction of fosterage and protection which we have seen to have such important and far-reaching consequences in the development of organic life. The storage of yolk in the ovum, the incubation of heavily yolked eggs, the self-sacrificing development in the womb, the elaboration of a supply of food-milk,—all these and other forms of fosterage may well have been the outcome of superabundant female vigour, the advantages of which are thus conferred upon the offspring.

We may now proceed to note, always remembering the paramount importance of the organism, some of the effects produced by changes in the environment.

The most striking and noteworthy feature about the effects of changes of climate and moisture, changes of salinity of the water in aquatic organisms, and changes of food-stuff, is that, when they produce any effect at all, they give rise to definite variations. Only one or two examples of each can here be cited. Mr. Merrifield,[EK] experimenting with moths (Selenia illunaria and S. illustraria), finds that the variations of temperature to which the pupa, and apparently also the larva, are subjected tend to produce "very striking differences in the moths." On the whole, cold "has a tendency, operating possibly by retardation, to produce or develop a darker hue in the perfect insect; if so, it may, perhaps, throw some light on the mechanism so often remarked in north-country examples of widely distributed moths." Mr. Cockerell[EL] regards moisture as the determining condition of a certain phase of melanism, especially among Lepidoptera. The same author states that the snail "Helix nemoralis was introduced from Europe into Lexington, Virginia, a few years ago. Under the new conditions it varied more than I have ever known it to do elsewhere, and up to the present date (1890) 125 varieties have been discovered there. Of these, no less than 67 are new, and unknown in Europe, the native country of the species." The effects of the salinity of the water on the brine-shrimp Artemia have already been mentioned. One species with certain characteristics was transformed into another species with other characteristics by gradually altering the saltness of the water. So, too, in the matter of food, the effects of feeding the caterpillars of a Texan species of Saturnia on a new food-plant were so marked that the moths which emerged were reckoned by entomologists as a new species.

The point, I repeat, to be especially noted about these cases and others which might be cited,[EM] is that the variation produced is a definite variation. Very probably it is generally, or perhaps always, produced in the embryonic or larval period of life. In some cases the variation seems to be transmissible, though definite and satisfactory proofs of this are certainly wanting. Still, we may say that if the changed conditions be maintained, the resulting variation will also be maintained. Under these conditions, at least, the variation is a stable one. It is probable that, apart from preferential mating, the varieties thus produced will tend to breed together rather than to be crossed with the parent form or varieties living under different conditions. In this way varieties may sometimes arise by definite and perhaps considerable leaps under the influence of changed conditions. We must not run the adage, Natura nil facit per saltum, too hard, nor interpret saltum in too narrow a sense.

It is true, and we may repeat the statement of the fact for the sake of emphasis, that we do not know how or why this or that particular variation should result from this or that change of climate, environment, or food-stuff; nor do we know why certain variations (such as that which produced the ancon breed of sheep) should be stable, while other variations are peculiarly unstable. But in this we are not worse off than we are in the study of inorganic nature. We do not know why calcite should crystallize in any particular one of its numerous varieties of crystalline form; we do not know why some of these are more stable than others. We may be able to point to some of the conditions, but we cannot be said to understand why arragonite should be produced under some circumstances, calcite under others; or why the same constituents should assume the form of augite in some rocks, and hornblende in other rocks. We are hedged in by ignorance; and perhaps one of our chief dangers, becoming with some people a besetting sin, is that of pretending to know more than we are at present in a position to know. Our very analogies by which we endeavour to make clear our meaning may often seem to imply an unwarrantable assumption of knowledge.

In the last chapter I used the term "organic combination," and drew a chemical analogy. I wished to indicate the particularity and the stability of certain variations, and the possibility of new departures through new combinations of variations, the new departure not being necessarily anything like a mean between the combining variations.[EN] I trust that this will not be misunderstood as a new chemico-physical theory of organic forms. I have some fear lest I should be represented as maintaining that a giraffe or a peacock is a definite organic compound, with its proper organic form, in exactly the same way as a rhombohedron of calcite or a rhombic dodecahedron of garnet is a definite chemical compound, with its proper crystalline form. All that the analogy is intended to convey is that variations seem, under certain circumstances, to be definite and stable, and may possibly combine rather than commingle.

Summary and Conclusion.

It only remains to bring this chapter to a close with a few words of summary and conclusion.

The diversity of animal life must first be grasped. We believe that this diversity is the result of a process or processes of evolution. Evolution is the term applied to continuity of development. It involves adaptation; and adaptation to an unchanging environment may become more and more perfect. But the environment to which organisms are adapted also changes. Where the change is in the direction of complexity, we have elaboration; where it is in the direction of simplicity, we have degeneration. Of these elaboration is the more important. It involves both a tendency to differentiation giving rise to individuality, and a tendency to integration giving rise to association. Continued elaboration is progress; and this is opposed to degeneration.

The factors of evolution fall under two heads—origin and guidance. The origin of variations lies in mechanical stresses, and chemical or physical influences. Whether these act on the body (and are transmitted by inheritance) or only on the germ, is a question which divides biologists into two schools. In the latter case all variations are fortuitous; in the former the development of tissue-variations has been in direct response to the physical or chemical influences. There are, however, in any case fortuitous combinations of variations.

Whether use and disuse are factors of origin is also a debatable point. Those who believe that physical influences on the body are transmissible believe also that the effects of use and disuse are transmissible.

The vital vigour of the organism is a determining condition of importance. The vital vigour of males has favoured the origin of secondary sexual characters; that of females, the fostering and protection of young, and therefore the development in them of vital vigour.

The almost universally admitted factor in guidance is natural selection. But we must be careful not to use it as a mere formula.

Whether sexual selection is also a factor is still a matter of opinion. Without it the specific character and constancy of secondary sexual features are at present unexplained. If inherited use and disuse are admitted as factors in origin, they must also be admitted as important factors in guidance.

Questions of origin and guidance should, so far as is possible, be kept distinct. These terms, however, apply to the origin and guidance of variations. In the origin of species guidance is a factor, no doubt a most important factor. The title of Darwin's great work was, therefore, perfectly legitimate. And those who say that natural selection plays no part in the origin of species are, therefore, undoubtedly in error.