302 THE EVOLUTION THEOKY 



caused to vary ^. The fact that we cannot recognize, for instance, 

 the beehive shape of an Ohha as an adaptation, is tlius no proof 

 that it is not one. But let us assume for the moment that it is not, 

 and that it cannot be referred to natural selection any more than the 

 other variations in the Celebes chain of forms, and we may further 

 admit that they cannot be referred to sexual selection, still less to 

 some ' inherent principle of perfecting,' not only because there is 

 no question of perfecting in the matter, but because such a mystical 

 principle is outside of the scope of natural history and its principles 

 of interpretation. 



But that transformations in a definite direction can and must 

 arise from fresh disturbances in the equilibrium of the determinant 

 system, that is from germinal selection, we have already shown. 



Even if the changes of form with which we are here dealing 

 had really no biological importance, they might quite well have been 

 brought about by germinal selection, and only one thing remains 

 obscure, that is, why the different stages on the path of distribution 

 of a species are at a different level of evolution and not all at the 

 same level. Why have not all been transformed 1 Why have some 

 colonies remained near the ancestral form, while others have varied 

 only a little, and others again a great deal "? This cannot be 

 explained by any assumption of an internal power of development, 

 and the explanation can only be found in germinal selection associated 

 with isolation, since the internal processes in the germ-plasm can 

 quite well run a different course in different colonies. Nevertheless 

 I am inclined to infer from these differences in the individual 

 colonies of these chains of forms, that natural selection in tlie 

 accepted sense has also played a part in the evolution of these 

 snail varieties. 



Such series of forms are especially interesting, because they 

 show us the process of species-formation in its different stages 

 beside each other in space, and thus simultaneously. They represent, 

 so to speak, a horizontal branch of the genealogical tree of the species, 

 as the Sarasins well express it, that is, a series of species arising from 



^ That this suggestion was not unjustified is evident from a recent contribution 

 by Simroth (' Ueber die Raublungenschnecken,' NaturwissenschaftUche Wockenschrift, 

 December 8 and 15, 1901). In this paper the author, who is an expert as regards 

 the biology of Gastropods, shows that a change of diet may evoke many kinds of 

 changes in the structure of the food-canal, which may indirectly compel changes in 

 the shell. Thus in a small indigenous snail, Daudehardia, the pharynx has grown so 

 enormously in thickness and length in adaptation to the predatory mode of life, that 

 the liead and the anterior part of the body can no longer be retracted within the 

 shelter of the shell. For this reason, and also because of the snail's habit of following 

 earthworms into their l)urrows, the shell has been shunted far back and obliquely 

 downwards. It has at the same time markedly changed in its shape, as may still be 

 verified by comparing the form of the shell in the young stages with that of tlie adult. 



