CONVERGENCE. 



153 



branches of the Chahna, not the converse ; and the forms 

 of Reniera likewise merge in species not constant in 

 any character, and which the most careful observer is 

 unable to distinguish from the Chalinula. Therefore, if 

 the convergence or approximation of branches of various 

 origin cannot be rejected in the abstract, the most pro- 

 pitious case of coincidence is, nevertheless, limited to the 

 province of analogous formations, where, under like con- 

 ditions of adaptation, different families have been driven 

 to like expedients and differentiations, producing com- 

 plete similarity. A general survey of the organic world 

 likewise teaches us, that in the higher regions this over- 

 lapping of the ends of dissimilar parentage becomes more 

 and more incredible, and, as is shown by my study on 

 sponges, they can, in any case, occur only where the 

 organisms consist of very simple factors, highly variable 

 in a few directions, and very easily affected by external 

 conditions. 



When we referred above to the possibility of no slight 

 objection to the doctrine of Descent, we spoke of another 

 case of convergence. We mean, namely, those similar 

 final results in divergent series by which, in highly or- 

 ganized groups of animals, of which the reciprocal con- 

 nection can be traced only through low aboriginal forms, 

 certain important organs exhibit the greatest uniformity 

 of arrangement and integration. It is, as yet, quite un- 

 decided where and when the true insects separated 

 themselves from the water-breathing crabs; nay, some 

 naturalists incline to the opinion that these two classes 

 are derived from a more remote common ancestor. 

 Thus much is extremely probable, that the severance 

 between crabs and insects took place when the develop- 



