HISTORICAL ZOOGEOGRAPHY 103 



commonly true of relict forms in the sea, where the physical conditions 

 have undergone relatively little change. The genus Nautilus, for ex- 

 ample, was a member of a flourishing family in the Mesozoic, but is 

 now the sole survivor of the tetrabranchiate cephalopods, and occurs in 

 only a few species in the Pacific and Indian oceans. The few modern 

 genera of pentacrinids are confined for the most part to deep seas and 

 represent a group of the Jurassic and Cretaceous. The few modern 

 ganoid fishes have become adapted to fresh water, where they have 

 escaped the brunt of the struggle for existence with the more modern 

 bony fishes, which have supplanted the ancestral ganoids in the sea. 

 Whatever the reasons for the occurrence of relict groups, they indicate 

 that some sort of change in their physical or biotic environment has 

 taken place. 11 



The satisfactory explanation of the discontinuity of distribution of 

 certain animals by demonstrable changes in the environment, as in the 

 glacial and steppe relicts, makes it possible to conclude in other in- 

 stances that present discontinuity must be based on changes in a for- 

 mer continuum. It becomes probable, in the case of such discontinuity 

 of related groups, that barriers to dispersal of some sort have arisen, 

 such as mountain ranges, deserts, climatic changes, rise of superior 

 competing forms, or extensions of the ocean. In respect to marine life, 

 bodies of land separating formerly continuous parts of the sea play the 

 same role in producing discontinuous or vicariating distributions. 



The mammalian faunae of Eurasia and North America exhibit a 

 high degree of similarity, in spite of the fact that the parts of the sea 

 which separate them are impassable barriers for the larger animals. 

 The number of genera in common is large, and the species of the same 

 genus are frequently so closely allied that they were formerly regarded 

 as subspecies of the same form. The beaver, elk, reindeer, and bears 

 are thus closely related, while the wisent and bison, the lynxes, and 

 the various deer, are only a little more distinct. This resemblance ex- 

 tends to such Tertiary animals as horses and camels, some of which 

 are now extinct in one or both areas. The supposition that a land 

 connection existed between the two continents in geologically recent 

 times, and that such a connection must have existed at various times 

 in the Tertiary, thus acquires a high degree of probability. 12 This 

 land bridge probably existed at Bering Strait, and may have included 

 part of the Arctic Ocean. The shallow seas in this region favor this 

 hypothesis. The great similarity between the fauna of Great Britain 

 and that of central Europe leads to a similar conclusion, and this 

 hypothesis is extensively supported by geological evidence. 



