118 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



sible to other forms. When a land connection is of short duration, 

 rapidity of spread becomes an important factor. Of twenty-two species 

 of amphibians and reptiles on the neighboring continent, thirteen 

 reached the British Isles after the glacial period, but only five of these 

 reached Ireland, which was cut off at an earlier date. Other factors 

 have contributed to the inequalities in dispersal. When a new land 

 bridge is established, the emigration of animals from the connected 

 regions will begin in both directions. Newcomers in each region come 

 into competition with the resident forms, and some will be able to 

 maintain themselves through adaptability and superiority of organiza- 

 tion, while others will be unable to get a foothold. As a rule, a land 

 connection will lead to a mutual exchange of forms rather than to a 

 one-sided invasion of one region from the other. The emigrations of 

 the South and North American faunae via the Panama bridge supply 

 the classic example of this phenomenon. 



The distribution of many groups will have the same limits, espe- 

 cially when there are effective barriers. New Zealand, Madagascar, 

 and the Hawaiian Islands are well isolated. Even in such places, the 

 different groups of animals are not differentiated from their relatives 

 in other regions to an equal degree. Since their isolation, the groups in 

 the separated regions may have evolved in different ways and at dif- 

 ferent rates. On the other hand, non-plastic groups, like many of the 

 invertebrates, may have undergone little change. The mammals of 

 Madagascar, for example, for the most part represent endemic families, 

 while among amphibians, reptiles, and insects, numerous genera are 

 the same as those of the African mainland. Australia is highly peculiar 

 in its mammalian fauna, while its lizards, butterflies, and earthworms 

 belong with those of the Oriental region. 



The faunal regions are therefore divisions of the earth's surface in 

 which the animal life bears a somewhat uniform aspect, and differs 

 from that of the neighboring regions in consequence of independent 

 evolution during longer or shorter periods of isolation. It is one of 

 the problems of zoogeography to study the history of the colonization 

 of the several regions and their subdivisions. In the course of this 

 inquiry it will usually be found that the present fauna of an area is 

 heterogeneous both as to the age and the origin of its various compo- 

 nents. 61 The course and direction of the dispersal of the various groups 

 and their order in time must be discovered. These relations may be 

 very different for different parts of the same region. 



Celebes and Ceylon both belong to the so-called Oriental 

 Region, according to the accepted scheme. But how totally different 

 has been their history! Celebes is a geologically young island, 



