FAUNA OF AMERICA — CLARK 289 



GEOLOGY AND ZOOLOGY 



These geological changes were accompanied by more or less exten- 

 sive changes in climate over the areas affected. Most important of 

 these changes were the climatic variations in North America. Before 

 and during the Tertiary a large part of North America was subtrop- 

 ical or tropical, and the southern part remained tropical until at 

 least the late Tertiary. This gradual change from a subtropical and 

 tropical to a temperate climate had the effect of restricting to the south, 

 or eliminating, completely, many of the elements in the original North 

 American fauna, a number of which are now found only in tropical 

 America. With the geographical and climatic changes, various major 

 and minor centers of evolution appeared throughout the Americas, giv- 

 ing rise to more or less distinctive types which spread to other areas, 

 became progressively restricted in their distribution, or disappeared. 



The geological history of the different vertebrate groups must be 

 considered in connection with their present distribution. First to 

 appear were the fishes, known from the Ordovician; amphibians are 

 first found in the Upper Devonian, reptiles in the Pennsylvanian, 

 and mammals probably in the Upper Triassic. The first known birds 

 are from the Jurassic. Although the genetic rate of evolution may 

 be independent of environment, an important factor in the evolution 

 of animal types is the relative stability or instability of the habitat 

 and its effect on natural selection. The basic ecology of the several 

 vertebrate groups varies to a greater or lesser degree, though there 

 is much overlapping, particularly in specilaized forms. Aquatic 

 habitats are the least variable, and so the fresh-water fishes, especially 

 in the Tropics, have the most generalized distribution with the nearest 

 approach to their distribution in the far-distant past. The distribu- 

 tion of the amphibians is, in general, parallel with that of the fishes. 

 As a result of the great variability of terrestrial conditions — diurnal 

 and seasonal — at different times in the past, the distribution of the 

 reptiles and especially of the mammals reflects present conditions 

 much more closely than docs that of the aquatic and amphibious 

 groups. In the reptiles, birds, and mammals those types that feed 

 entirely, or almost entirely, in water are in general more widely and 

 generally distributed than those that are entirely terrestrial. 



Worthy of special mention in the present fauna of America are such 

 very ancient types as the ganoid fishes of North America, the lungfishes 

 and osteoglossids of South America, various frogs of South America, 

 and the ribbed or tailed frog {Ascaphus) of our Northwest related 

 to Liopelma of New Zealand, and among invertebrates the onycho- 

 phores {Peripatus and its relatives) of Central and South America 

 and the West Indies. In North America the paddlefish (Polyodon) 

 and the alligator, each with a single relative in China ; the hellbender 



