108 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



the species of Pliocene snails are still extant. The reasons for this more 

 rapid evolution of the warm-blooded animals are doubtless to be found 

 in the general acceleration of the life processes, including the appear- 

 ance of mutations, and in the greater probability of the survival of 

 new forms owing to their adaptability to environments which are closed 

 to other groups of animals. The degree of differentiation between re- 

 lated groups of mammals thus gives a clue to the amount of time since 

 their origin, and consequently, in some cases, to the date of union of 

 land areas now separated. Thus the presence of so many genera of 

 mammals in Eurasia and North America represented by closely allied 

 species or even subspecies in the two regions is the basis for the belief 

 that these regions were connected in geologically recent times. Even a 

 separation since the Pliocene, from the evidence of the extinct Pliocene 

 forms, would have resulted in a greater differentiation than exists. 



The rapid transformation undergone by the species of warm-blooded 

 animals explains the fact that the faunae of Madagascar, South Amer- 

 ica, and Australia are so much more peculiar in these groups than in 

 their insects or amphibians and reptiles. Celebes, as an example, has no 

 endemic genera of land mollusks, 3 among the fresh-water mollusks, 

 1 for amphibians and reptiles, but harbors 12 peculiar genera of birds 

 and 3 of mammals. 



Finally, the fossil remains of extinct forms are especially abundant 

 among the mammals, more abundant at any rate than among other 

 terrestrial animals. The study of this palaeontological evidence affords 

 reliable information about former connections of land areas, and some- 

 times even of separations, as was illustrated above in the discussion of 

 the relation between the North and South American faunae. 



Other animals besides mammals furnish similar evidence of former 

 changes in the distribution of land. Land connections between areas 

 now separated but inhabited by related animals need not have been 

 direct; they may involve other areas, and may have been successive 

 rather than continuous at any one time. The more ancient the groups 

 of animals whose discontinuous distribution requires explanation, the 

 greater are the possibilities for their migration by a roundabout route. 

 The occurrence of two genera of mantids, Liturgousa and Stegmatop- 

 tera 2S in Madagascar and South America, and nowhere else, and the 

 similar case of the insectivores, the Centetidae and Solenodontidae in 

 Madagascar and the West Indies, cannot be supposed to indicate direct 

 connection between South America and Madagascar. Allied fossils have 

 been found in the North American Eocene. 



Numerous examples are known of a modern discontinuous distribu- 

 tion which is supplemented by a wider distribution of fossil remains. 



