104 ZOOLOGY. 



means that the production of many individuals, a large num- 

 ber of which never reach maturity, secures the development of 

 a small aristocracy which propagates the type. The species 

 is related to its individuals something as the individual is to 

 the renewed and changing cells of which it is composed. 

 Species are not constant, but even the most fixed must undergo 

 change or extinction when confronted by new conditions. 

 Species however are less variable than the individuals com- 

 posing them because the species represents an average con- 

 dition of all its individuals. Adaptation to environment is the 

 great problem which every animal must solve. Those which 

 do solve it successfully constitute the species. It is needful 

 then to consider next those characteristics of structure, habit, 

 or instinct whereby a species of organisms becomes success- 

 fully adjusted to its surroundings. In a broad sense all the 

 organs which were outlined in the preceding chapters are adap- 

 tations : the digestive organ and process, to the nature of food ; 

 the nervous system and the special senses, to the external 

 stimuli; the lungs, gills, and skin to the need of oxygen, and 

 the like. In contrast to adaptations of this kind we now con- 

 sider as adaptations those more special modifications of fun- 

 damental structure by which a species becomes more suited 

 to some limited habitat or to some special mode of life which 

 is of signal use to it in the struggle for existence. 



141. Classification of the Principal Types of Adaptation. 



A. Adaptations primarily in relation to the inorganic en- 

 vironment. 



B. Adaptations primarily related to other organisms. 

 I. Among animals of the same species. 



1. Friendly and social, 

 (a) Mating. 



(&) Parental care of young. 



(c) Organic colonies. 



(d) Social and communal life. 



2. Competitive : for food, mates, etc. 



