DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 105 



and has allowed these to reproduce, eliminating the others. 

 This artificial process, which obtains results more rapidly than 

 the natural, has given rise to the various breeds, strains, or 

 races of dogs, horses, cattle, fowls, etc. By means of this selec- 

 tion the habits and dispositions of the domestic animals have 

 been improved as surely as their structure. Their power of 

 self-support, however, has been so materially diminished that 

 some of them could not succeed in finding a living in the wild 

 state under ordinary circumstances. 



139. Practical Exercises. Are there any domesticated animals whose species 

 is represented in the wild state? Compare the habits and general structure of 

 some of the domesticated animals with that of their nearest kin among wild species. 

 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 unlimited 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 

 adapting itself 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 



