We have seen that certain insects are possessed of warning colours, which advertise their nastiness to the taste. Birds avoid these bright but unpleasant insects, and though there is some individual learning, there seems to be an instinctive avoidance of these unsavoury morsels. There is hesitation before tasting; and one or two trials are sufficient to establish the association of gaudiness and nastiness. Moreover, Mr. Poulton and others have shown that, under the stress of keen hunger, these gaudy insects may be eaten, and apparently leave no ill effects. Birds certainly instinctively avoid bees and wasps; and yet the sting of these insects can seldom be fatal. It is, therefore, improbable that nastiness or even the power of stinging can have been an eliminating agency. In the development of the instinctive avoidance, natural selection through elimination seems to be excluded, and the inheritance of individual experience is thus rendered probable. As before pointed out, it is not enough to say that a nasty taste or a sting in the gullet is disadvantageous; it must be shown that the disadvantage has an eliminating value. From my experiments (feeding frogs on nasty caterpillars, and causing bees to sting chickens), I doubt the eliminating value in this case. Hence elimination by natural selection seems, I repeat, to be excluded, and the inheritance of individual experience rendered probable.

Mr. Herbert Spencer has contended that, in certain modifications, natural selection is excluded on the grounds of the extreme complexity of the changes, and adduces the case of the Irish "elk" with its huge antlers, and the giraffe with its specially modified structure. He points out that in either case the conspicuous modification-the gigantic antlers or the long neck-involves a multitude of changes affecting many and sometimes distant parts of the body. Not only have the enormous antlers involved changes in the skull, the bones of the neck, the muscles, blood-vessels, and nerves of this region, but changes also in the fore limbs; while the long neck of the giraffe has brought with it a complete change of gait, the co-ordinated movements

of the hind limbs sharing in the general modification. Mr. Spencer, therefore, argues that it is difficult to believe that these multitudinous co-ordinated modifications are the result of fortuitous variations seized upon by natural selection. For natural selection would have to wait for the fortunate coincidence of a great number of distinct parts, all happening to vary just in the particular way required. That natural selection should seize upon the favourable modification of a particular part is comprehensible enough; that two organs should coincidently vary in favourable directions we can understand; that half a dozen parts should, in a few individuals among the thousands born, by a happy coincidence, vary each independently in the right way is conceivable; but that the whole organization should be remodelled by fortunately coincident and fortuitously favourable variations is not readily comprehensible. It may be answered-Notwithstanding all this, we know that such happy coincidences have occurred, for there is the resulting giraffe. The question, however, is not whether these modifications have occurred or not, but whether they are due to fortuitous variation alone, or have been guided by functional use. The argument seems to me to have weight.

Still, we should remember that among neuter antsfor example, in the Sauba ant of South America (Oecodoma cephalotes) there are certain so-called soldiers with relatively enormous heads and mandibles. The possession of these parts so inordinately developed must necessitate many correlated changes. But these cannot be due to inherited use, since such soldiers are sterile.

Furthermore, according to Professor Weismann, natural selection is really working, not on the organism at large, but on the germ-plasm which produces it; and it is con

Mr. Darwin, while contending that the modifications need not all have been simultaneous, says, “Although natural selection would thus tend to give the male elk its present structure, yet it is probable that the inherited effects of use, and of the mutual action of part on part, have been equally or more important" ("Animals and Plants under Domestication," vol. ii. p. 328).

ceivable that the variation of one or more of the few cells in early embryonic life may introduce a great number of variations in the numerous derivative cells. In explanation of my meaning, I will quote a paragraph from a paper of Mr. E. B. Poulton's on "Theories of Heredity. "It appears," he says, "that, in some animals, the great groups of cells are determined by the first division [of the ovum in the process of cleavage t]; in others, the right and left sides, or front and hind ends of the body; while the cells giving rise to the chief groups on each side would then be separated at some later division. This is not theory, but fact; for Roux has recently shown that, if one of the products of the first division of the egg of a frog be destroyed with a hot needle, development is not necessarily arrested, but, when it proceeds, leads to the formation of an embryo from which either the right or the left side is absent. When the first division takes place in another direction, either the hind or the front half was absent from the embryo which was afterwards produced. After the next division, when four cells were present, destruction of one produced an embryo in which one-fourth was absent." Now, it is conceivable that a single modification or variation of the primitive germ might give rise to many correlated modifications or variations of the numerous cells into which it develops; just as an apparently trivial incident in childhood or youth may modify the whole course of a man's subsequent life. It is difficult, indeed, to see how this could be effected; to understand what could be the nature of a modification of the germ which could lead simultaneously to many favourable variations of bones, muscles, blood-vessels, and nerves in different parts of the body. This, however, is a question of the origin of variations; and it is, at any rate, conceivable that, just as by the extirpation with a hot needle of one cell of the cleaved frog's ovum all the anterior part of the body should be absent in development, so by the appropriate modification of this one cell, or the germinal matter which produced it, * Midland Naturalist, November, 1889. † See ante, p. 52.

all the anterior part of the body should be appropriately modified.

These considerations, perhaps, somewhat weaken the force of Mr. Spencer's argument, which is not quite so strong now as it was when the "Principles of Biology" was published.

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(2) We may pass now to the evidence afforded by direct observation and experiment. There is little enough of it. The best results are, perhaps, those which have been incidentally reached in the poultry-yard and on the farm in the breeding of domesticated animals. We have seen that, under these circumstances, certain parts or organs. have very markedly diminished in size and efficiency; others have as markedly increased. Of the former, or decrease in size and efficiency," the imbecile ducks with greatly diminished brains have been already mentioned. Mr. Herbert Spencer draws attention to the diminished efficiency in ear-muscles, giving rise to the drooping ears of many domesticated animals. "Cats in China, horses in parts of Russia, sheep in Italy and elsewhere, the guineapig formerly in Germany, goats and cattle in India, rabbits, pigs, and dogs in all long-civilized countries, have dependent ears." Since many of these animals are habitually well fed, the principle of economy of growth seems excluded. Indeed, the ears are often unusually large; it is only their motor muscles that have dwindled either relatively or absolutely. If what has been urged above be valid, panmixia cannot have been operative; since panmixia per se only brings about regression to mediocrity. If the effects. in these two cases, ducks' brains and dogs' ears, be not due to disuse, we know not at present to what they are due. In the correlative case of increase by use, we find it exceedingly difficult to exclude the disturbing effects of artificial selection. The large and distended udders of cows, the enhanced egg-laying powers of hens, the fleetness or strength of different breeds of horses,-all of these have been *Nature, vol. xli. p. 511.

"Animals and Plants under Domestication," vol. ii. p. 291.

subjects of long-continued, assiduous, and careful selection. One cannot be sure whether use has co-operated or not.

Sufficient has now, I think, been said to show the difficulty of deciding this question, the need of further observation and discussion, and the necessity for a receptive rather than a dogmatic attitude; and sufficient, also, to indicate my reasons for leaning to the view that use and disuse, long-continued and persistent, may be a factor in organic evolution.

The Nature of Variations.

The diversity of the variations which are possible, and which actually occur in animal life, is so great that it is not easy to sum up in a short space the nature of variations. Without attempting anything like an exhaustive classification, we may divide variations into three classes.

1. Superficial variations in colour, form, etc., not necessarily in any way correlated with

2. Organic variations in the size, complexity, and efficiency of the organs of the body;

3. Reproductive and developmental variations.

Any of these variations, if sufficient in amount and value to determine the question of elimination or not-elimination, selection or not-selection, may be seized upon by natural selection.

Our domesticated animals exemplify very fully the superficial variations which, through man's selection, have in many cases been segregated and to some extent stereotyped. It is unnecessary to do more than allude to the variations in form and coloration of dogs, cattle, fowls, and pigeons. These variations are not necessarily in any way correlated with any deeper organic variations. They are, however, in many cases so correlated. For example, the form of the pouter pigeon is correlated with the increased size of the crop, the length of the beak carries with it a modification of the tongue, the widely expanded tail of the fantail carries with it an increase in the size and number