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Show 236 INSTINCT. CHAP. VII. for their similarity by inheritance from a common parent, and must therefore believe that they have been acquired .by independent acts of natural selection. I will not here enter on these several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities : for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. The subject well deserves to be discussed at great length, but I will here take only a single case, that of working or sterile ants. How the workers have been rendered sterile is a difficulty ; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile ; and if such insects had been social, and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, I can see no very great difficulty in this being effected by natural selection. But I must pass over this preliminary difficulty. The great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax and in being destitute of wings and sometimes of eyes, and in instinct. As far as instinct alone is concerned, the prodigious difference in this respect between the workers and the perfect females, would have been far better exemplified by the hive-bee. If a working ant or other neuter insect had been an animal in the ordinary state, I should have unhesitatingly assumed that all its characters had been slowly acquired through natural selection; namely, by an individual CHAP. VII. NEUTER INSECTS. 237 ha~ing been born with some slight profitable modification of structure, this being inherited by its off: . · h . h · . sp11ng, w Ic again vaned and were again selected and d B .h ' so o~wa: s. ut Wit the working ant we have an insect drffenng .greatly from its parents, yet absolutely sterile; so t~at It co.uld ~ever have transmitted successively acquued modifications of structure or instinct to its progeny. It may well be asked how is it possible to reconcile this case with the theory of natural selection? . First, let it b~ remembered that we have innumerable I.n stances, bof th In our domestic productions and in tho se In a state o nature, of all sorts of differences of struc-ture. which have become correlated to certain ages, and to mther sex. We have differences correlated not onl to one sex, but to that short period alone when the r!product~ ve systen: is active, as in the nuptial plumage of many buds, and In the hooked jaws of the male salmon. We have even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the male sex; for oxen of certain breeds have longer horns than in other breeds, in comparison with the horns of the bulls or cows of these same breeds. He~ce I can see no real difficulty in any character ha v~g become correlated with the sterile condition of c.ert~In n1embers of insect-communities : the difficulty hes In understanding how such correlated modifications of structure could have been slowly accumulated by natural selection. This difficulty, though appearing insuperable is lessened, or, as I believe, disappears, when it is' remembered that selection may be applied to the family ~s :veil as to the individual, and may thus gain th~ esued end. Thus, a well-flavoured vegetable is cooked and the individual is destroyed ; but the horticulturist sows seeds of the same stock, and confidently expects to I • |