9& ZOOLOGY. 



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 selection the habits and dispositions of the 

 domestic animals have been improved as surely as their struc- 

 ture. Their power of self-support, however, has been so ma- 

 terially diminished that some of them could not succeed in 

 finding a living in the wild state under ordinary circumstances. 



135. 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 ? 



136. 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 ( 132) . This struggle is not merely among 

 the animals in question, but is in reality between every organ- 

 ism 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 mi- 

 grate to regions where the conditions are naturally more favor- 

 able 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 



