ORGANIZATION TENDS TO ADVANCE. 113 



his law "that homologous units of any order become differ- 

 entiated 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 produced : variations 

 in a single species inhabiting an isolated station might be 

 beneficial, and thus the whole mass of individuals might 

 be modified, or two distinct forms might arise. But, as I 

 remarked toward 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 pro- 

 found ignorance on the mutual relations of the inhabitants 

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

 past ages. 



CONVERGENCE OF CHARACTER. 



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

 portance of divergence of character (in which, however, 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 convergence 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 forces, and it is not surprising that dissimilar sub- 

 stances should sometimes assume the same form ; but with 

 organic beings we should bear in mind that the form of each \ 

 depends on an infinitude of complex relations, namely on , 

 the variations which have arisen, these being due to causes ' 

 far too intricate to be followed out — on the nature of the 

 variations which have been preserved or selected, and this 

 depends on the surrounding physical conditions, and in a 

 still higher degree on the surrounding organisms with which 

 each being has come into competition — = and lastly, on inher- 

 itance (in itself a fluctuating element) from innumerable 

 progenitors, all of which have had their forms determined 



