Chap. IV.] CONVERGENCE OF CHARACTER. 101 



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 surround- 

 ing physical conditions, and in a still higher degree on the sur- 

 rounding organisms with which each being has come into competi- 

 tion, — and lastly, on inheritance (in itself a fluctuating element) from 

 innumerable progenitors, all of which have had their forms Aeter- 

 mined 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 organi- 

 sation. 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 con- 

 ditions are concerned, it seems probable that a sufficient number 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 conditions of life 

 must become more and more complex. Consequently there seems 

 at first sight no limit to the amount of profitable diversification of 

 structure, and therefore no limit to the number of species which 

 might be produced. We do not know that even the most prolific 

 area is fully stocked with specific forms : at the Cape of Good Hope 

 and in Australia, which support such an astonishing number of 

 species, many European plants have become naturalised. But 

 geology shows us, that from an early part of the tertiary period the 

 number of species of shells, and that from the middle part of this 

 same period the number of mammals, has not greatly or at all 

 increased. What then checks an indefinite increase in the number 

 of species? The amount of life (I do not mean the number of 

 specific forms) supported on an area must have a limit, depending 

 so largely as it does on physical conditions ; therefore, if an area be 

 inhabited by very many species, each or nearly each species will be 

 represented by few individuals ; and such species will be liable to 

 extermination from accidental fluctuations in the nature of the 

 seasons or in the number of their enemies. The process of extermi- 

 nation in such cases would be rapid, whereas the production of new 



