CHAP. IV.] CONVERGENCE OF CHARACTER. 95 



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, <fec. ; 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 repre- 

 sented 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 always- 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. Rare species, and each species will become 

 rare if the number of species in any country becomes indefinitely 

 increased, will, on the principle often explained, present within a 

 given period few favourable variations ; consequently, the process 

 ot giving birth to new specific forms would thus be retarded. 

 When any species becomes very rare, close interbreeding will 

 help to exterminate it ; authors have thought that this comes into 

 play in accounting for the deterioration of the Aurochs in 

 Lithuania, of Red Deer in Scotland, and of Bears in Norway, &c. 

 Lastly, and this I am inclined to think is the most important 

 element, a dominant species, which has already beaten many 

 competitors in its own home, will tend to spread and supplant 

 many others. Alph. de Candolle has shown that those species 

 which spread widely, tend generally to spread very widely; 

 consequently, they will tend to supplant and exterminate several 

 species in several areas, and thus check the inordinate increase of 

 specific forms throughout the world. Dr. Hooker has recently 

 ehown that in the S.E. corner of Australia, where, apparently, 



