WALLACE'S ESSAY. 73 



physically unchanged, the numbers of its animal 

 population cannot materially increase." If one species 

 does so, others must diminish. In the immense 

 amount of destruction the weakest must die, " while 

 those that prolong their existence can only be the 

 most perfect in health and vigour those who are 

 best able to obtain food regularly and to avoid their 

 numerous enemies. It is, as we commenced by re- 

 marking, 'a struggle for existence,' in which the 

 weakest and least perfectly organised must always 

 succumb." 



This tendency must apply to species as well as 

 individuals, the best adapted becoming abundant, the 

 others scarce or even extinct. If we knew the whole 

 of the conditions and powers of a species " we might 

 be able even to calculate the proportionate abundance 

 of individuals, which is the necessary result." 



Hence, first, the animal population of a country 

 is generally stationary (due to food and other 

 checks); second, comparative abundance or scarcity 

 of individuals is entirely due to organisation and 

 resulting habits, the varying measure of success in 

 the struggle being balanced by a varying population 

 in a given area. 



Variations from type must nearly always affect 

 habits or capacities. Even changes of colour may 

 promote concealment, while changes in the limbs 

 or any external organs would affect the mode of 

 procuring food, etc. "An antelope with shorter or 

 weaker legs must necessarily suffer more from the 



