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tile 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 between 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 genera of the same family。 Thus in Eciton； there are working and soldier neuters； with jaws and instincts extraordinarily 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 Myrmecocystus； the workers of one caste never leave the nest； they are fed by the workers of another caste； and they 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； which our European ants guard or imprison。
It will indeed be thought that I have an overweening confidence in the principle 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 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 proportionally 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 males 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 in the same 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 my 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 differed 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 widely…different 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 selection； by acting on the fertile parents； could form a species which should regularly produce neuters； either all of large size with one form of jaw； or all of small size with jaws having a widely different structure； or lastly； and this is our climax of difficulty； one set of workers of one size and structure； and simultaneously another set of workers of a different size and structure； a graduated series having been first formed； as in the case of the driver ant； and then the extreme forms； from being the most useful to the community； having been produced in greater and greater numbers through the natural selection of the parents which generated them； until none with an intermediate structure were produced。
Thus； as I believe； the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest； both widely different from each other and from their parents； has originated。 We can see how useful their production may have been to a social community of insects； on the same principle that the division of labour is useful to civilised man。 As ants work by inherited instincts and by inherited tools or weapons； and not by acquired knowledge and manufactured instruments； a perfect division of labour could be effected with them only by the workers being sterile； for had they been fertile； they would have intercrossed； and their instincts and structure would have become blended。 And nature has； as I believe； effected this admirable division of labour in the communities of ants； by the means of natural selection。 But I am bound to confess； that； with all my faith in this principle； I should ne