No. 643] 



EFFECTS OF ENVIRONMENT 



147 



may survive. The little spider that spins a cocoon under 

 stones guards her treasure with watchful care and, if 

 she is compelled to leave her cocoon, spins a ground line 

 as she runs in order that she may return without delay. 

 A male spider dances, postures, and uses all his arts to 

 secure a mate. As soon as he has mated, Nature usually 

 sacrifices his life to his offspring— for his hard-earned 

 mate devours him if she can. 



Thus it is wherever one considers animals. There is 

 adjustment, frequently of a very specialized type, to en- 

 vironment. The wonder of it all is the degree of adapta- 

 tion that animals show. In speaking of food relations 

 Semper^ said, "there is scarcely a constituent of the 

 earth's crust, whether on land or in water— not an animal 

 nor a plant, whether living, dead, or even in decomposi- 

 tion—which does not afford nourishment to some living 

 animal." The first more or less self-evident generaliza- 

 tion justified by this discussion may be stated as follows: 

 Animals are adapted to the environment. 



That animals are adapted, probably no one disputes, but 

 there has been much controversy as to the means by 

 which they have become adapted. There appear to be 

 three effects that it is possible for the environment to 

 produce in animals : (1) a direct transformation or modi- 

 fication of the living system of activities, (2) the de- 

 struction of systems unsuited to the environment and the 

 ''survival of the fittest," and (3) the migration of sys- 

 tems from unfavorable to favorable environments. 



Animals are modified by external changes and may even 

 take on different forms to suit different environments. 

 Sponges and corals growing in deep water usually have 

 a branching form ; the same species in shallow water form 

 flat, encrusting growths. The brine-shrimp, Artemia 

 salina, is a classical instance of an animal that has many 

 forms, and these are rather closely correlated with the 

 salinity of the water in which it lives. Sumner" and Shel- 



