114 CONVERGENCE OF CHARACTER. 



through equally complex relations. It is incredible that the 

 descendants of two organisms, which had originally differed 

 in a marked manner, should ever afterward converge so 

 closely as to lead to a near approach to identity through- 

 out their whole organization. If this had occurred, we should 

 meet with the same form, independently of genetic connec- 

 tion, 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 number of species would soon 

 become adapted to all considerable diversities of heat, 

 moisture, etc. ; 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 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 naturalized. 

 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) sup- 

 ported 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 extermination in such cases would 

 be rapid, whereas the production of new species must alwa}^s 

 be slow. Imagine the extreme case of as many species as 

 individuals in England, and the first severe winter or very 

 dry summer would exterminate thousands on thousands of 

 species, Bare species, and each species will become rare if 



