THE ORIGIN OF SPECIES 



Had a filter bridge, let alone a corridor, existed, a much broader repre- 

 sentation of each order should have entered. Thus the carnivores of Africa 

 include a wide variety of cats, but only the related viverrids reached 

 Madagascar. The Primates of Africa include many monkeys and apes, 

 but none of these have reached Madagascar. Only two ungulates, out of 

 a large African ungulate fauna, reached Madagascar. These are a bush- 

 pig and a pigmy hippopotamus which is now extinct. 



PROBLEMATICAL DISTRIBUTIONS 



Discontinuous distributions of many species may appear at first inspection 

 difficult to understand. As geological data are assembled, these frequently 

 become readily understandable on the basis of corridors or filter bridges 

 which are no longer extant. Of course, if the bridge is still extant, there 

 is no difficulty at all. But some distributions can be understood only on 

 the assumption that the discontinuity was established via a sweepstakes 

 route. These cases are likely to remain in dispute even after thorough 

 study, because many biologists are unable to accept the reality of so in- 

 determinate a route. They prefer to look for land connections, and then 

 search for some factor that would prevent its use by a broad sampling of 

 the floras and faunas. The great difficulties to which this has at times led 

 may well indicate the artificiality of the goal. 



Cain lists no less than seventeen pairs of areas between which major 

 discontinuities exist. He does not regard his list as exhaustive, but sug- 

 gests that other authors may wish to extend it. Only a few more examples 

 will be discussed below. 



The Eastern Asia-Eastern North America Case. When the biotic 

 similarities of eastern North America and eastern Asia were first discov- 

 ered, an explanation was by no means easily given. Here were two closely 

 similar biotic regions separated by nearly half of the circumference of 

 the globe and a great ocean, yet they had many genera and even some 

 species in common! But geological and paleontological studies revealed 

 that these regions were broadly continuous during the Mesozoic Era and 

 during much of the Tertiary period. The climate was much warmer, and 

 thus these temperate and subtropical plants and animals were able to 

 inhabit the great space now separating them. Details of their subsequent 

 separation have been discussed above. 



Bipolar Mirrorism. Du Reitz has studied what he calls bipolar "mirror- 

 ism" of floras of the northern and southern hemispheres, in which the 

 same or closely related species may be present in the temperate and 

 boreal zones of both hemispheres. Thus he finds a botanical correspond- 

 ence between the Mexican plateau and Peru; between Texas and Argen- 

 tina; between Chile and California; and between the Straits of Magellan 

 and Arctic North America. The plants which he studied are principally 

 mosses, lichens, sedges, grasses, and other plants the species of which are 

 not generally familiar to laymen, but some examples may be cited. The 

 crowberry shrul), Empetrum, is found both in North America and in the 

 Andes. Alder trees, Ahius, are found in the mountains of Mexico and 



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