456 COMPARATIVE ZOOLOGY 



more agility than other members of the herd would be 

 better able to escape his enemies, and consequent^ to 

 live longer and leave more offspring than his less 

 fortunate companions. Of his progeny some would 

 probably inherit the peculiarity, and it would thus be 

 transmitted from generation to generation to the evi- 

 dent advantage of the race. In time the variation 

 would become a definitely fixed and constant character, 

 serving to distinguish all the individuals possessing it as 

 a species. It is by a process of artificial selection that 

 breeders choose among domestic animals those indi- 

 viduals which possess a character which it is desired to 

 perpetuate, as long wool in sheep, speed in race horses, 

 strength in draught horses, peculiarly shaped jaws in 

 bull dogs, vocal powers in canary birds, and so on. By 

 breeding only from those individuals which show the 

 desired character, the latter may not only be perpetu- 

 ated but also intensified in degree. This is shown by 

 all domesticated animals and cultivated plants. To the 

 process by which favorable variations are selected and 

 perpetuated among wild animals, Darwin gave the name 

 of natural selection. By his hypothesis the phenomenon 

 of the evolution of organic forms is due to the natural 

 selection of favorable variations and their preservation 

 by heredity. 



The test of the validity of a theory lies in its ability 

 to interpret and coordinate observed facts, and when 

 Darwin's hypothesis was applied to the elucidation of 

 the observations collected by the students of morphol- 

 ogy, paleontology, embryology, and other aspects of 

 the study of animal life, it brought order out of chaos, 

 and each of these sciences was seen to contain a mass 

 of evidence in favor of the theory of descent with 

 modification. 



(i) Evidence from Classification. As has been shown 



