VERTEBRATES — CLARK 303 



temperate regions of the Northern Hemisphere, however, the waters 

 are now all cold, at least in winter. Especially is this true in North 

 America where the Mississippi drains an area with a more or less 

 severe winter climate, and the present fishes are almost wholly of 

 types not represented in South America or in other tropical regions. 



The close affinities of the fish fauna of South America with those 

 of Africa and the Indian region are most logically explained by 

 assuming that all three are local modifications of a very ancient and 

 generally distributed fauna persisting under conditions that have 

 here changed but little since the Paleozoic. In their distribution the 

 fresh-water fishes show many cases of apparently anomalous dis- 

 tribution that can be explained only by restriction of a formerly 

 widespread range. For example, the lungfishes are now confined 

 to Africa, South America, and Australia; the family Polyodontidae 

 is represented only in the Mississippi Valley (Polyodon) and in the 

 large Chinese rivers (Psephurus) ; the shovel-nosed sturgeons (Scaph- 

 irhynchus) are confined to the Mississippi area and central Asia; the 

 North American catfishes (Ameiuridae) are confined to North Amer- 

 ica except for a single species in China; the suckers (Catostomidae) 

 are wholly North American except for two species in eastern Asia; 

 and the mud minnows ( Umbra) are confined to North America except 

 for a single species in the Danube Valley in Europe. 



Another type of relict distribution is represented by those fishes 

 of which only those that have adapted themselves to life in under- 

 ground waters still persist. Among these are Lucifuga (Brotulidae) 

 of the caves of Cuba and the Amblyopsidae with four genera and seven 

 species in the subterranean waters of the eastern half of the Mississippi 

 Valley, and one genus with two species of normally appearing fishes 

 in the coastal swamps from Virginia to Georgia. 



The gar pikes (Lepidosteus) known from as far back as the Ter- 

 tiary in America and Europe still occur in North and Central America 

 and Cuba, and another very ancient type, the bowfin {Amid) lives 

 in North America. 



Considering animals as a whole, the vertebrates represent a rela- 

 tively small, closely knit group of large or very large types with 

 exceptional power of adaption to changing conditions, most highly 

 developed in the mammals and birds, the two classes that were the 

 last to appear and that have departed most widely from the original 

 vertebrate stock, though in such a way as to be complementary to 

 each other rather than direct competitors. The present distribution 

 of the vertebrates appears to be mainly the end result of a series of evo- 

 lutionary adaptations to changing conditions in each class, within 

 their ecological limitations, the most ecologically limited classes, and 

 also the oldest, the fishes and amphibians, still retaining the closest 

 approach to vertebrate distribution in the Tertiary. 



