But this change in the mode of rearing bee-larvæ gives rise to pure queens and not to mixed forms, and we must therefore regard it as undecided whether Wasmann's explanation is correct in this case, and whether an abnormality in the constitution of the germ-plasm may not be the true cause of this or other kinds of mixed forms among ants. In any case the 'Lomechusa hypothesis' rests upon the assumption of different kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and the differences between the worker and queen-ants have their cause in this, and not directly in the kind of larval food. If there were not different ids corresponding to the different kinds of individuals in the germ-plasm a kind of polymorphism might indeed have arisen in the colony through differences in nutrition, but it could not have been of the kind we now see--that is, a sharply defined differentiation of persons, in adaptation to their different functions. This presupposes elements in the germ which can vary slowly and consistently in a definite direction without causing any change in the rest of the germ. This state of affairs gives to the phyletic evolution of the workers a great theoretical significance, for it proves that positive as well as negative variations of the most diverse parts of the body, that simultaneous and correlative variations of many parts, can take place in the course of the phylogeny, without the co-operation of the Lamarckian factor. I have not hitherto laid any special emphasis upon the degree of differences occurring between workers and queens; but I must now add that this may far exceed the degree that we are familiar with in our common indigenous ants, both in regard to instinct and to bodily form. Even in the red Amazon ant of Western Switzerland, _Polyergus rufescens_, we find quite a new instinct[17], that of carrying off the pupæ of other species of ants, not to devour, but to introduce them to their own nest and thus secure 'slaves.' For these workers of a strange species, which emerge in a strange nest, naturally regard the place of their birth as their home, and do there what instinct impels them, and what they would have done in the nest of their parents: they feed the larvæ, fetch food, collect building material, and so on. The domestic activity of the workers of the master-species thus becomes superfluous, and they have ceased to exercise it, and have now entirely lost the power of caring for their brood, searching for food, and keeping up the nest. They have even forgotten how to take food themselves, because they are always fed by the 'slaves.' Forel informs us--and I have myself repeated the experiment--that _Polyergus_ workers, which are shut up with a drop of honey on the floor of their prison, will leave it, their favourite food, untouched, and finally starve, unless one of their 'slaves' be shut up with them. As soon as this happens, and the slave perceives the