EVOLUTION-PAST AND PRESENT 



643 



only the animals that survived but also the 

 direction evolution took in any given lo- 

 cality. 



Although there seems to be some ques- 

 tion about the movement of entire conti- 

 nents (continental drift), there is no doubt 

 about the fact that there have been many 

 "ups and downs" within the various conti- 

 nents themselves. The presence of fossil 

 remains of sea animals on the top of moun- 

 tains is unequivocal evidence that these 

 regions were at the bottom of some ancient 

 ocean. Great sections located in mid-conti- 

 nental North America abound in marine 

 fossils, though these areas are over a 

 thousand miles from the sea today. The 

 rising and falling of these areas has oc- 

 curred again and again in certain regions, 

 and geologists can tell us how many times 

 and approximately when this has happened. 

 There is no doubt but that such striking dis- 

 locations of great areas had a profound 

 effect on animal life, and we know the 

 history of some of these. 



Coincident with these shifts in the sur- 

 face of the earth, the climate changed dra- 

 matically from one extreme to another and 

 back again. The great coal deposits in the 

 northern sections of the globe are a result 

 of luxuriant plant growth in some ancient 

 period, which is substantial proof that such 

 regions were tropical during these times. 

 Areas that once received a tremendous 

 rainfall later became dry plains. This transi- 

 tion can be observed today by flying over 

 large regions of the North Central United 

 States. It is possible to see the outlines of 

 ancient lakes that are now forest lands and 

 old forest lands that have become grass- 

 lands. All of these changes have come 

 about because of the gradual trend toward 

 a more arid climate. The sudden appear- 

 ance of a mountain range must have had a 

 profound effect on the amount of precipi- 

 tation that fell in surrounding regions, 

 which in turn would alter both plant and 

 animal life. Undoubtedly the success of a 

 species depends a great deal upon the co- 



incidence of gene mutations with changes 

 in the climate and topography of the re- 

 gion. The proper timing of genetic changes 

 in the animal with climatic and environ- 

 mental changes might mean the success or 

 failure of a species. These constant shifts in 

 climate and topography have influenced the 

 evolution of animals to a marked deeree. 



Closely related species of animals are 

 usually found in adjacent ranges (region 

 occupied by a species), just as one might 

 expect. If they had split off from one an- 

 other only recently, they would not be sepa- 

 rated by very great distances, and more than 

 likely some barrier would exist between 

 them such as a mountain, canyon, or body 

 of water, which may have been responsible 

 for the new species in the first place. The 

 more recent the separation of two species 

 the more closely related they are, and the 

 more ancient the separation the greater the 

 differences between them. The greater dif- 

 ferences constitute genera, families, orders, 

 classes, and phyla. When great land masses 

 are separated for millions of years one 

 would expect the entire fauna to be quite 

 different on each of them. Such is found to 

 be the case when, for example, the animals 

 in Australia and in the continent of Asia 

 are compared. The explanation of the oc- 

 currence of primitive animals in Australia 

 has direct bearing on the history of this 

 large land mass. 



Most of the earth's land is located in the 

 northern hemisphere. It was all connected 

 at one time when there was a land bridge 

 across Bering Strait. Therefore, animals 

 originating in any part of this great land 

 mass could have migrated to any other part 

 during this period. It is thought that mam- 

 mals originated in these northern conti- 

 nents and spread in all directions. The 

 frigid barrier to the north encouraged 

 southward migrations of primitive mam- 

 mals during the Mesozoic when the three 

 great land masses to the south (Africa, 

 South America, and Australia) were con- 

 nected at various times to the continents 



