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Show 212 INSTINCT. [CHAP. VII they have an enormously developed abdornen which se.~ cretes a sort of honey, supplying the place of that excreted by the aphides, or the do1nestic cattle as they may be called, which our European ants guard or imprison. It will jndeed be thought that I have an overweening confidence in the p1~nciple 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 fro1n the fertile males and fmnales,-in this case we 1nay safely conclude from the analogy of ordinary variations, that each successive, slight, pl'ofitable Jnodification did not pTobably 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 EuTope have been carefully examined. MT. 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 sa1ne nest: I have myself compared perfect gradations of this kind. It often happens that the larger or the srnaller sized workers are the most numerous; or that both large and small are nuinerous; with those of an intermediate size scanty in numbers. Formica flava has larger and smaller workers, with some of intermediate size; and, in this species, as Mr. F. Smith has observed, the larger workers have simple eyes (ocelli), which though small can be plainly distinguished, whereas the smaller workers have their ocelli rudimentary. Having carefully dissected several specimens of these workers, I can affirm that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their pro- OnAP. VII.] NEUTER INSECTS. 213 portionally lesser size; and I fully believe, though I dare not assert so positively, that the workers of intermediate size have their ocelli in an exactly intermediate condition. So that we here have two bodies of sterile workers in the same nest, differing not only in size, but in their organs of vision, yet connected by some few members in an intermediate condition~ I may digress by adding, that if the smaller workers had been the most useful to the community, and those 1nales and females had been continually selected, which produced more and more of the smaller workers, until all the workers had come to be in this condition; we should then have had a species of ant with neuters very nearly iri the sarne condition with those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli, though the male and female ants of this genus have well-developed ocelli. . I may give one other case: so confidently did I expect to find gradations in important points of structure between the different castes of neuters in the same species, that I gladly availed myself of Mr. F. Smith's offer of numerous specimens from the same nest of the driver ant (Anomma) of West Africa. The reader will perhaps best appreciate the amount of difference in these workers, by n1y giving not the actual measurements, but a strictly accurate illustration : the difference was the same as if we were to see a set of workmen building a house, of whom many were five feet four inches high, and many sixteen feet high; but we must suppose that the larger workmen had heads four. instead of three times as big as those of the smaller men, and jaws nearly five times as big. The jaws, moreover, of the working ants of the several sizes differ wonderfully in shape, and in the form and number of the teeth. But the important fact for us is, that though the workers can be grouped into castes of different sizes, yet they graduate insensibly into each other, as does the widelydifferent structure of their jaws. I speak confidently on this latter point, as Mr. Lubbock made drawings for me with the. camera lucida of the jaws which I had dissected from the workers of the several sizes. With these facts before me, I believe that natural se- |
