The Ecology, Evolution, and Distribution 

 of the Vertebrates 



By Austin H. Clark 



Associate in Zoology, U. S. National Museum 



The geological history and the accompanying evolution of the ver- 

 tebrates are the products of the interaction of a large number of vari- 

 ous factors the final results of which are seen in the diversity, 

 distribution, and interrelationships of the several classes that we see 

 at the present time. These factors are both geological, in the broad 

 sense, and biological. The geological factors may be briefly sum- 

 marized as follows: 



Ever since the first appearance of the vertebrates, conditions on the 

 land areas have undergone constant and continual change. Warm 

 lands have become cool or even cold, while cold lands have become 

 warm, especially in the Northern Hemisphere, as, for instance, in 

 areas affected by the advance and retreat of glaciers. Well-watered 

 lands have become arid and arid lands fertile. Seasonal and diurnal 

 variations in temperature and humidity have increased or decreased. 

 Mountains have arisen, greatly affecting conditions in their immedi- 

 ate vicinity, especially on their leeward sides, and other mountains 

 have been eroded down. Land areas have been partially submerged, 

 resulting in separate land masses or islands of various sizes, and iso- 

 lated lands have been joined. Isolation by the dismemberment of an 

 extensive land area, or the appearance of other barriers, provides an 

 opportunity for such terrestrial animals as may still be able to find 

 adequate support to form new centers of evolution and to develop new 

 subtypes, while the joining of previously separated land masses, as, 

 for instance, North and South America, or North America and Asia 

 during the Pleistocene, permits the exchange of faunas, with result- 

 ing competition. 



These changes are continuous. The well-known floral and faunal 

 changes in North America as a result of the advance and retreat of 

 the great glaciers are a familiar example. Even within historic 

 times we have records of far-reaching changes, especially in North 

 Africa, the Near East, and India, and comparable changes are now 



283 



