PALEONTOLOGY 317 



tolerate the new conditions, and followed the retreating ocean. If they 

 did not migrate, they perished. 



Smaller changes in the environment, especially those occurring on land 

 such as a slight change of climate or of vegetation, probably presented 

 less rigorous alternatives. As was pointed out in the preceding chapter, 

 individual animals of today may become slightly altered structurally 

 or physiologically so as to be capable of living in a habitat from which 

 their unmodified relatives are barred. If these changed individuals live 

 near the margin of their range they may migrate into some adjoining area 

 where the other members of their species cannot follow them. Thus a 

 new species, occupying a contiguous region, may arise. The same princi- 

 ple may have operated in the past, permitting migration to occur. Such 

 modifications in animals as would fit them for slightly different environ- 

 ments may occasionally, where the environment itself was being altered, 

 also have operated in a different manner and made migration unneces- 

 sary. There are some instances of ancient terrestrial animals which ap- 

 pear to have become modified almost simultaneously with changes in 

 their environment. If while a region was being slowly altered as to cli- 

 mate, soil, vegetation or other features, certain individual animals within 

 the region were modified in a way which happened to fit them for the 

 new conditions, the changed individuals could remain while the unaltered 

 members of their species might be forced to migrate or might perish. 

 Some biologists hold that, in case of a simultaneous change in the environ- 

 ment and in the animals, the change of environment caused the change 

 in the animals. But there is much in favor of the view that the two 

 changes were wholly independent of one another in origin. In the cases of 

 parallel alteration of animals and environment, the two changes happened 

 to fit each other. We are ignorant of those (perhaps much more numer- 

 ous) cases in which the changes of animals and environment did not suit 

 each other, for the animals must have perished or migrated early in the 

 period of modification. 



Migration of animals has also been helped or hindered by the forma- 

 tion or destruction of land bridges connecting continents, etc. Reference 

 to these geological changes was made in the preceding chapter to explain 

 peculiarities of the present geographical distribution of animals. Paleon- 

 tology also affords evidence of such changes. Fossils of horses in Europe 

 and America indicate a former connection between those continents. 

 North and South America were connected up to middle Eocene time (Fig. 

 217, A) and were then separated (B) until late in the Miocene epoch or 

 later when, as indicated by fossils of mastodons, deer, horses, etc., mi- 

 gration between the two continents again became possible. 



Migration of the Environment. — In the alteration of climate, topog- 

 raphy, etc., a given kind of environment is believed to have traveled, 

 in some instances, across a continent during the passage of time. Case 



