DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION III 



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. Spe- 

 cies 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 composing 

 them because the species represents an average condition 

 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 in- 

 stinct whereby a species of organisms becomes successfully ad- 

 justed to its surroundings. In a broad sense all the organs 

 which were outlined in the preceding chapters are adaptations : 

 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 consider as 

 adaptations those more special modifications of fundamental 

 structure by which a species becomes more suited to some lim- 

 ited habitat or to some special mode of life which is of signal use 

 to it in the struggle for existence. 



145. 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. 



(6) Parental care of young. 



(c) Organic colonies. 



(d) Social and communal life. 



2. Competitive: for food, mates, etc. 



II. Among animals of different species. 

 i. Friendly and social. 



(a) Commensalism. 

 (6) Symbiosis. 



