68 



ISLAND LIFE. 



[part I. 



has been named Heliodus by Professor Newberry. Thus an 

 enormous range in time is accompanied by a very wide and 

 scattered distribution of the existing species. 



Whenever, therefore, we find two or more living genera be- 

 longing to the same family or order but not very closely allied 

 to each other, we may be sure that they are the remnants of a 

 once extensive group of genera ; and if we find them now 

 isolated in remote parts of the globe, the natural inference is 

 that the family of which they are fragments once had an area 

 embracing the countries in which they are found. Yet this 

 simple and very obvious explanation has rarely been adopted 

 by naturalists, who have instead imagined changes of land and 

 sea to afford a direct passage from the one fragment to the 

 other. If there were no cosmopolitan or very wide-spread 

 families still existing, or even if such cases were rare, there 

 would be some justification for such a proceeding ; but as about 

 one-fourth of the existing families of land mammalia have a 

 range extending to at least three or four continents, while many 

 which are now represented by disconnected genera are known 

 to have occupied intervening lands or to have had an almost 

 continaous distribution in tertiary times, all the presumptions 

 are in favour of the former continuity of the group. We have 

 also in many cases direct evidence that this former continuity 

 was effected by means of existing continents, while in no single 

 case has it been shown that such a continuity was impossible, 

 and that it either was or must have been effected by means of 

 continents now sunk beneath the ocean. 



Concluding Remarks. — When writing on the subject of dis- 

 tribution it usually seems to have been forgotten that the 

 theory of evolution absolutely necessitates the former existence 

 of a whole series of extinct genera filling up the gap between 

 the isolated genera which in many cases now alone exist ; while 

 it is almost an axiom of " natural selection " that such nume- 

 rous forms of one type could only have been developed in a 

 wide area and under varied conditions, implying a great lapse 

 of time. In our succeeding chapters we shall show that the 

 known and probable changes of sea and land, the known 

 changes of climate, and the actual powers of dispersal of the 



