184 THE ORIGIN OF SPECIES 



The amount of life (I do not mean the number of specific 

 forms) supported on an area must have a limit, depend- 

 ing so largely as it does on physical conditions; there- 

 fore, if an area be inhabited by very many species, each 

 or nearly each species will be represented by few indi- 

 viduals; 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 ex- 

 termination 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 indidviuals 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 in- 

 creased, will, on the principle often explained, present 

 within a given period few favorable variations; conse- 

 quently, 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 Lithu- 

 ania, of Red Deer in Scotland, and of Bears in Norway, 

 etc. 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 



