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, "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, "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‡ 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 "Animals and Plants under Domestication," vol. ii. p. 244. † “Darwinism,” p. 293. "Evolution of Sex," p. 22. 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,* experimenting with moths (Selenia illunaria and S. illustraria), finds that the variations of temperature to which the pupæ, and apparently also the larvæ, 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 * "Incidental Observations in Pedigree Moth-breeding," F. Merrifield, Transactions Entomological Society, 1889, pt. i. p. 79, et seq. the perfect insect; if so, it may, perhaps, throw some light on the melanism so often remarked in north-country examples of widely distributed moths." Mr. Cockerell* 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,† 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 * Nature, vol. xli. p. 393. + See Professor Meldola's edition of Professor Weismann's "Studies in the Theory of Descent," and Mr. Cunningham's translation of Professor Eimer's" Organic Evolution." 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. I trust that this will not be misunderstood as a new chemicophysical 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 * See Darwin, " Animals and Plants under Domestication,” vol. ii. p. 252. 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 R |