DIFPERENTIATION OF INDIVIDUALS AND ADAPTATION 107 



How many species of domestic animals can you enumerate? From what groups 

 do they come? Trace the history and results of the domestication of some of the 

 common animals, as fowls, pigeons, cats, dogs, etc. Have any strictly American 

 species been domesticated? 



140. The Adaptation of Animals to their Environment. — 



There are two distinct questions of importance to be con- 

 sidered in connection with this subject: (i) the necessity of the 

 adjustment of organisms to their environment, and (2) the 

 means by which this adaptation takes place in the individual and 

 becomes fixed in the species. It is clear that the limited food 

 supply and the high powers of animals to reproduce result in 

 a struggle for food among the animals at any time occupying 

 the earth (135). This struggle is not merely among the animals 

 in question, but is in reality between every organism and its 

 whole environment. Extremes of heat and cold, drouth and 

 famine, and numerous changes in the conditions of life make it 

 absolutely necessary that the individual shall have some power of 

 becoming adapted to what is permanent and what is changeable 

 in its environment. What are the means then by which animals 

 that are not completely in accord with their surroundings may 

 become so? There are two possible ways in which this may 

 come about. The animals may migrate to regions where the 

 conditions are naturally more favorable to their well-being, that 

 is, to regions for which they are already adapted. As a matter 

 of fact this is known to be a common occurrence. Animals 

 often disperse from their old centre of multiplication under 

 the influence of hunger or other unfavorable local conditions. 

 They are often assisted in these dispersals by such natural 

 agencies as winds, currents of water, and other animals. If the 

 migrating forms succeed in finding new regions suited to their 

 needs, there results a condition of adaptation between organ- 

 isms and their respective environments, but without any active 

 change in the characteristics of the organism. The environ- 

 ment itself is subject to continual change and there are too 

 many barriers in the way of universal migration for this to be 

 accepted as a complete explanation of the widely observed 

 adjustment of animals to the conditions which surround them. 

 Again animals may become suited to their environment by 



