GENERAL ZOOLOGICAL FACTS AND THEORIES 293 



toes, and a corresponding increase in the 

 size of one toe, until an animal, Equus, is 

 evolved with one toe on each foot and the 

 splintlike remains of what were functional 

 toes in its Eocene ancestor. 



(d) Geographical Distribution. The 

 facts of geographical distribution all seem 

 to show that species originate in some par- 

 ticular place, from which they disperse as 

 their numbers increase. When groups are 

 prevented from mingling with others of 

 their kind by environmental barriers or 

 physiological conditions, the factor of isola- 

 tion, combined with other factors, result in 

 the evolution of new geographical races and 

 finally new species. Accordingly, when 

 species that resemble each other closely are 

 found occupying neighboring regions, but 

 separated by barriers, we conclude that 

 they have evolved from a common ances- 

 tor, and have diverged until their differ- 

 ences are considered of specific rank. 



THE THEORY OF NATURAL SELECTION. - 

 The origin of species, as conceived by 

 Darwin, is largely due to natural selection. 

 We may gain a clear idea of what is meant 

 by natural selection if we examine the re- 

 sults of artificial selection. For example, 

 there are at the present time over one hun- 

 dred and fifty different domestic varieties of 

 pigeons; a few of these are shown in Figure 



FIG. 159. Diagrams illustrating the gradual changes 

 in foot structure in fossil and recent species of 

 the horse family, a, bones of the fore foot ; 

 b, bones of the hind foot. (From Metcalf after 

 Marsh.) 



