310 



HABIT AND INTELLIGENCE. 



[chap. 



Struggle 



for 



existence. 



Eaces 

 probably 

 mix little 

 in nature. 



Eapicl 

 increase. 



or removing barriers to migration. Besides — as Darwin 

 was, I believe, the first to state, with any emphasis 

 proportioned to the importance of the fact — any change 

 in the distribution of one species is almost certain 

 to produce changes in the conditions of the life of 

 other species, by altering the character and abundance 

 of their food, the character and number of their enemies, 

 and the character of the species with which they have 

 to compete for a subsistence. The importance of this 

 element of competition, or " struggle for existence " (to 

 quote again from Darwin's title-page), was also first in- 

 sisted on by Darwin. I shall have to return to this 

 subject shortly. It is very difficult to say how much 

 effect is to be ascribed to mixture of races occurring in a 

 state of nature. Among plants the pollen of one is carried 

 to the stigma of another, by the wind or by insects, quite 

 at random ; and in this way races may be mixed. But, 

 for reasons to be explained further on, it is not common 

 for two varieties of the same species to be found in the 

 same habitat, and this must tend to prevent mixture. The 

 same is tme among animals ; and besides, among the 

 higher animals, which have a mental nature and a power 

 of choice, it seems certain that this will be exercised in a 

 way that will keep races separate.^ For these reasons, I 

 am inclined to think that there is very little mixture of 

 races in a state of nature, and that the stimulus to varia- 

 tion is chiefly given by changes in the circumstances of life. 



Having spoken of these preliminary questions, it is 

 time to state the manner in which natural selection will 

 take place. 



All organisms multiply so rapidly that when any geolo- 

 gical or climatic change opens a new habitat to a species, a 

 time which is geologically very short will suffice to stock 

 the new habitat with as large a population of that species 

 as it can support ; and when this has been done, many more 



1 This is true of domestic races. When any degree of freedom is per- 

 mitted, different breeds of the same species, as sheep or pigeons, prefer to 

 herd and to breed each apart. (Darwin on Variation under Domestication, 

 Tol. ii. chapter xvi.) 



