iNcisuEA (scissurella) lytteltonensis. 41 



examples of detailed resemblances which cannot be due to 

 inheritance nor yet can they be due to external forces acting 

 upon homogenetic parts, for the teeth are formed below the 

 gum and the cusps are in place before any mechanical forces 

 are brought to bear on them. The characters of the teeth 

 are clearly congenital, and the resemblances between the 

 patterns which have arisen independently in different groups 

 cannot be accounted for by the preservation of fortuitous 

 variations by natural selection, for paleeontological evidence 

 shows that variation has in each case proceeded along one 

 line and not along several lines, one of which has been 

 selected. 



Calliug to mind Lankester's suggestion of the "common 

 action of evoking causes ... on parts which for other 

 reasons (than homogeny) offer a likeness of material to begin 

 with," Osborn pleads for the recognition of a latent or 

 potential homology, by which term I understand him to mean 

 a tendency or capacity to produce a definite structure, which 

 capacity must have been present in the ancestors of the 

 existing orders of Mammalia, but has only manifested itself in 

 such groups as possessed or were subject to the co-operating 

 factors necessary for evoking the latent capacity, and thus 

 producing the structure in question. 



The objections to a principle of this kind are that, in the 

 first place, as Osboim himself admits, it leads us on the 

 dangerous ground of teleological speculation ; and, in the 

 second place, that it might, if loosely applied, be used to 

 explain anything or everything by a phi-ase. 



Nevertheless, I think that some such principle may be 

 admitted, with due caution, in explanation of a large number 

 of difficulties which present themselves, with increasing 

 insistence, to every class of zoological workers. In a i-ecent 

 paper on the Neritid^ I alluded to the great difficulty of 

 finding a satisfactory theory to account for the distribution of 

 the fresh-water Neritids, described as species of the sub- 

 genera Paranerita and Septaria, in remote oceanic islands. 

 As their general anatomical and conchological characters 



