CHAPTER V. 



SOME THEORETICAL CONSIDERATIONS. 



The bearing of the facts of distribution upon the 

 places of origin of different groups. 



There seems to be little doubt that life first originated 

 in the. sea, whence it spread to the fresh waters and so to 

 the land. We are not however concerned in the present 

 volume, which only professes to deal with the distribution 

 of terrestrial and a few fresh-water groups, with this 

 problem. 



But it is necessary to see how far the existing and 

 past facts of distribution enable the place of origin of a 

 particular terrestrial group to be indicated with prob- 

 ability. No exhaustive survey of the different groups of 

 the animal kingdom will be attempted ; but a few facts 

 and suggested explanations will be put forward as 

 examples. 



In some cases,, of course, palaeontology throws a clear 

 light upon the problem. The horse, for instance, came 

 into being in the new world, whence it migrated into the 



