DISTRIBUTION OF SPECIES AND GENERA. l8l 



tralia, while a single species inhabits West Africa. 

 And other good examples might be given, all of 

 which, from the fact that they are considered re- 

 markable, are to be regarded as exceptions to the 

 rule which finds genera confined to definite, con- 

 tinuous localities. 



Taking families now into consideration, we are 

 prepared to find a much greater amount of disconti- 

 nuity. Families are only highly developed genera, 

 but so long ago was it that all the forms of a given 

 family had a common starting-point, that the vari- 

 ous genera will have had ample opportunity to 

 wander perhaps all over the world ; to have become 

 greatly modified ; to have completely disappeared 

 in some localities, being displaced by other animals 

 better adapted to the conditions of life ; and even, 

 perhaps, to have become separated by impassable 

 barriers, which have arisen since the ' migrations, 

 such as recent mountain ranges. We should there- 

 fore expect that some families may have an almost cos- 

 mopolitan range, particularly those with great powers 

 of migration. It is hardly necessary to say that the 

 facts bear out this expectation. Some families of 

 bats and birds have a practically world-wide distri- 

 bution ; many have a remarkably discontinuous area 

 at the present day, although fossils tell us that for- 

 merly they were widely distributed. The camels 

 are divided by the Atlantic Ocean, one genus living 

 in Africa, and a second in South America. One 

 family of insectivora is found in Madagascar, Cuba, 

 and Hayti. On the other hand, there are a few 

 examples of restricted family areas. The birds of 



