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Show 238 INSTINCT. CHAP. VII. get nearly the same variety ; breeders of cattle wis~ the flesh and fat to be well marbled together ; th~ animal has been slaughtered, but the breeder goes :"Ith. confidence to the same family. I have such faith In the p· owers of selecti"on , that I do n. ot doubt th. at a. breed of cattle, always yielding oxen with extraordinanly l~ng horns, could be slowly formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns ; and yet no one ox could ever have propagated its kind. Thus I believe it has been with social insects : a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community : consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification. And I believe that this process has been repeated, until that prodigious amount of difference ~etween the fertile and sterile females of the same species has been produced, which we see in many social insects. But we have not as yet touched on the climax of the difficulty ; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. The castes, moreover, do not generally graduate into each other, but are perfectly well defined; being as distinct from each other, as are any two species of the same genus, or rather as any two gener~ of the same family. Thus in Eciton, there are worki~g a~d soldier neuters, with jaws and instincts extraordinanly different : in Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite unknown : in the Mexican M yrme- CHAP. VII. NEUTER INSECTS. 239 cocystus, the workers of one caste never leave the nest · they are fed by the workers of another caste, and the; have an enormously developed abdomen which secretes a sort of honey, supplying the place of that excreted by the. aphides, or the domestic cattle as they may be called, wluch our European ants guard or imprison. It will in_deed be ~ho~ght that I have an overweening confidence In the prmmple of natural selection, when I do not admit that such wonderful and well-established facts at once annihilate my theory. In the simpler case of neuter insects all of one caste or of the same kind, which have been rendered by natural selection, as I believe to be quite possible, different from the fertile males and females,-in this case, we may safely conclude from the analogy of ordinary variations, that each successive, slight, profitable modification did not probably at first appear in all the individual neuters in the same nest, but in a few alone; and that by the long-continued selection of the fertile parents which produced most neuters with the profitable modification, all the neuters ultimately came to have the desired character. On this view we ought occasionally to find neuter-insects of the same species, in the same nest, presenting gradations of structure ; and this we do find, even often, considering how few neuter-insects out of Europe have been carefully examined. Mr. F. Smith has shown how surprisingly the neuters of several British ants differ from each other in size and sometimes in colour; and that the extreme forms can sometimes be perfectly linked together by individuals taken out of the same nest : I have myself compared perfect gradations of this kind. It often happens that the larger or the smaller sized workers are the most numerous; or that both large and small are numerous, with those of an intermediate size scanty in numbers. Formica flava has larger and |