CHARLES DARWIN 241 



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 in- 

 dividuals ; and such species will be liable to exterminate 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 Eng- 

 land, and the first severe winter or very dry summer would extermi- 

 nate thousands on thousands of species. Rare species, and each spe- 

 cies 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 of 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 inordi- 

 nate increase of specific forms throughout the world. Dr. Hooker 

 has recently shown that in the S. E. corner of Australia, where, appar- 

 ently, there are many invaders from different quarters of the globe, 

 the endemic Australian species have been greatly reduced in num- 

 ber. How much weight to attribute to these several considerations 

 I will not pretend to say; but conjointly they must limit in each 

 country the tendency to an indefinite augmentation of specific 

 forms. 



