102 NATURAL SELECTION. [Chap. IV. 



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 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 Eed Deer in Scot- 

 land, 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 exter- 

 minate several species in several areas, and thus check the inordinate 

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

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

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

 the endemic Australian species have been greatly reduced in number. 

 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. 



Summary of Chapter. 



If under changing conditions of life organic beings present indivi- 

 dual differences in almost every part of their structure, and this 

 cannot be disputed ; if there be, owing to their geometrical rate of 

 increase, a severe struggle for life at some age, season, or year, and 

 this certainly cannot be disputed ; then, considering the infinite 

 complexity of the relations of all organic beings to each other and to 

 their conditions of life, causing an infinite diversity in structure, com 

 stitution, and habits, to be advantageous to them, it would be a most 

 extraordinary fact if no variations had ever occurred useful to each 

 being's own welfare, in the same manner as so many variations have 

 occurred useful to man. But if variations useful to any organic 

 being ever do occur, assuredly individuals thus characterised will 

 have the best chance of being preserved in the struggle for fife ; 

 and from the strong principle of inheritance, these will tend to 

 produce offspring similarly characterised. This principle of pre- 



