140 ORIGIN OF SPECIES 



quently 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 Aus- 

 tralia, 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 con- 

 ditions; therefore, if an area be inhabited by very many spe- 

 cies, 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 exter- 

 mination 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 favorable 

 variations; consequently, the process of giving birth to new 

 specific forms would thus be retarded. When any species be- 

 comes very rare, close interbreeding will help to exterminate 

 it ; authors have thought that this comes into play in account- 

 ing 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 :L-everal species 

 in several areas, and thus check the inordinate increase oi 



