132 ZOOLOGY. 



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 produc- 

 tion of the young. As it is sometimes expressed, nature sacri- 

 fices the individual to the welfare of the species. 



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

 make or mar their success in life. The more powerful the 

 influence the more certain the adaptation, because the destruc- 

 tion is the more certain in case of failure. The principal 

 classes of adaptations are, those relating to the using of the 

 favorable and resisting the unfavorable features of the inani- 

 mate environment; those assisting in the obtaining of food 

 whether vegetable or animal; those of mating and care of 

 young; those of offense and defense, in predaceous animals 

 and their prey. The relations and adaptations range all the 

 way from indifference to friendship, and from feeding at the 

 same talble on the one hand, to the utmost antagonism on the 

 other. 



9. Perhaps the most important and the least understood of 

 the series of adaptations which animals acquire are those con- 

 nected with the nervous system and its functions : the habits, 

 instincts, and intelligence of animals. They are inseparable 

 from those already enumerated, and yet in fundamental im- 

 portance they form a group of their own. They seem pri- 

 marily to depend upon the irritability of protoplasm which 

 enables it not merely to respond but to become permanently 

 changed by that response a kind of organic memory. From 

 this fact acclimatization and adjustment become possible. 



10. In being scattered from their starting place, animals 



