THE ORIGIN OF ANIMAL SPECIES 465 



retreat of the great ice sheet which, during the last gla- 

 cial period, covered much of Europe, Asia, and North 

 America, these boreal species were left stranded in, and 

 have since been confined to, regions having arctic char- 

 acteristics. Species which are closely related to one 

 another are known to inhabit mountain peaks, separated 

 by long stretches of lowland, on which animals of the 

 arctic type could not possibly exist. Their presence 

 can be explained only on the supposition that they are 

 descended from forms which inhabited the entire region 

 during the glacial period and that when the climate 

 became warmer these animals retreated to the cold 

 mountain tops. Many oceanic islands are destitute of 

 batrachians and terrestrial mammals, these animals not 

 having had an independent evolution in these localities, 

 nor being able to make their way out from the mainland. 

 On the other hand, aerial mammals, as bats, are of 

 nearly universal distribution. 



It is generally admitted that each species originated in 

 one locality and, by migration, spread into neighboring 

 regions, becoming modified as dispersal brought it into 

 different environments, thus giving rise to variations 

 which ultimately resulted in the development of a num- 

 ber of more or less closely related species. 



Such are the main features of the theory of evolution 

 and of Darwin's explanation of the process through va- 

 riation, heredity, and natural selection. As a subordinate 

 factor should be mentioned his theory of sexual selection, 

 by which is meant that the choice of mates, among the 

 higher animals at least, is largely determined by such 

 physical characteristics as strength, beauty of form, 

 coloration, and vocal powers. Those individuals, for 

 instance, which possessed any of these pleasing char- 

 acteristics in a higher degree than their companions 

 would be more likely to find mates and to leave de- 

 DODGE'S GEN. ZOOL. 30 



