138 ZOOLOGY 



ences in the environment. The environment may produce 

 very important changes during the single life of the individual. 



4. The food supply of animals is limited, since all ultimately 

 depend on plants; any species multiplying at its average rate 

 of increase could in a short time, if unchecked, stock the earth 

 up to its limits of support; that this does not occur is due to a 

 struggle for food among the excessive numbers which are 

 born, whereby only a small percentage of them reach maturity. 

 In the main, those survive which possess some qualities which 

 tend to fit them for the environment in which they find them- 

 selves. These are thus enabled to transmit their qualities to 

 their offspring, the fittest of which are again chosen. The 

 result is adaptation, and the process is known as natural 

 selection. 



5. A similar result is effected by man in domestic animals 

 by artificially selecting individuals in accordance with the pos- 

 session of certain features. The resulting forms are frequently 

 very unsuited to the natural environment, and could not survive 

 if left to themselves. 



6. As the result of various causes animals become dispersed 

 from their point of origin, and in becoming adapted to the 

 different regions into which they go, or through variation 

 within a given region, give rise to new varieties. When, by 

 any means, these groups have become perfectly adapted to 

 their new special environment and permanently different from 

 their parent stock and from each other, without intermediate 

 individuals which manifestly connect the varieties, they are 

 recognized as new species. Through the influence of heredity 

 and by natural selection these differences may accumulate, 

 apparently to any amount. 



7. The nutritive function relates particularly to the con- 

 tinued existence of the individual; the reproductive function 

 looks to the continuance of the species, and is a tax on the in- 

 dividual. Nature has specially favored those organisms in 

 which an increasing degree of energy is given to the production 

 and care of the young. As it is sometimes expressed, nature 

 sacrifices the individual to the welfare of the species. 



8. Animals become adapted to all the influences that tend to 



