CONVERGENCE OF CHARACTER 139 



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 at- 

 tribute to convergence a close and general similarity of struc- 

 ture 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 substances 

 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 varia- 

 tions 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 inheritance (in it- 

 self a fluctuating element) from innumerable progenitors, 

 all of which have had their forms determined through equally 

 complex relations. It is incredible that the descendants of 

 two organisms, which had originally differed in a marked 

 manner, should ever afterwards converge so closely as to lead 

 to a near approach to identity throughout their whole organ- 

 isation. If this had occurred, we should meet with the same 

 form, independently of genetic connection, recurring in 

 widely separated geological formations ; and the balance of 

 evidence is opposed to any such an admission. 



Mr. Watson has also objected that the continued action 

 of natural selection, together with divergence of character, 

 would tend to make an indefinite number of specific forms. 

 As far as mere inorganic conditions are concerned, it seems 

 probable that a sufficient numJier of species would soon 

 become adapted to all considerable diversities of heat, 

 moisture, &c. ; but I fully admit that the mutual relations of 

 organic beings are more important ; and as the number of 

 species in any country goes on increasing, the organic con- 

 ditions of life must become more and more complex. Conse- 



