CONVERGENCE, 153 
branches of the Chalina, 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 
conditions of adaptation, different families have been 
driven to like expedients and differentiations, producing 
complete similarity: A general survey of the organic 
world likewise teaches us that in the higher regions this 
overlapping 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 organized groups of animals, of which the 
reciprocal connection can be traced only through low 
aboriginal forms, certain important organs exhibit the 
greatest uniformity of arrangement and integration. 
It is, as yet, quite undecided 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 
