156 COXVEEGEXCE OE CHAEACTEE. [Chap. IT- 



homologous units of anv 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 isolated 

 station might he beneficial, and thus the whole mass of 

 individ uals 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 species, 

 if we make due allowance for our profound ignorance 

 on the mutual relations of the inhabitants of the world 

 at the present time, and still more so during past ages. 



Convergence of CJiaracter. 



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 



