156 CONVBRaBNCE OF CHARACTER. [Chap. IV. 



homologous units of any order become differentiated in 

 proportion as their relations to incident forces become 

 different " would come into action. But as we have no 

 facts to guide us, speculation on the subject is almost 

 useless. It is, however, an error to suppose that there 

 would be no struggle for existence, and, consequently, 

 no natural selection, until many forms had been pro- 

 duced: variations in a single species inhabiting an iso- 

 lated station might be beneficial, and thus the whole 

 mass of individuals might be modified, or two distinct 

 forms might arise. But, as I remarked towards the close 

 of the Introduction, no one ought to feel surprise at 

 much remaining as yet unexplained on the origin of spe- 

 cies, if we make due allowance for our profound igno- 

 rance on the mutual relations of the inhabitants of the 

 world at the present time, and still more so during past 



Convergence of Character. 



Mr. H. C. Watson thinks that I have overrated the 

 importance of divergence of character (in which, how- 

 ever, he apparently believes), and that convergence, as 

 it may be called, has likewise played a part. If two 

 species, belonging to two distinct though allied genera, 

 had both produced a large number of new and divergent 

 forms, it is conceivable that these might approach each 

 other so closely that they would have all to be classed 

 under the same genus; and thus the descendants of two 

 distinct genera would converge into one. But it would 

 in most cases be extremely rash to attribute to con- 

 vergence a close and general similarity of structure in 

 the modified descendants of widely distinct forms. The 

 shape of a crystal is determined solely by the molecular 



