CHAPTER III. 



THE CAUSES WHICH INFLUENCE THE DISTRIBUTION 



OF ANIMALS. 







Distribution not dependent upon temperature. 



IT was at one time held that distribution depended 

 upon temperature ; that therefore the world could be 

 divided into zones corresponding to the belts of varying 

 temperature. 



That the range of animals is to a large degree depen- 

 dent upon temperature is an undoubted fact; and to a 

 certain extent that fact does permit of the zonal arrange- 

 ment of the earth. Only however as concerns the arctic 

 regions ; here we have occasionally the same species 

 ranging right round the pole as we have in the case of 

 the marine mammalia and the birds of the south pole. 

 Some even go so far as to unite for the same reason the 

 Palaearctic and Nearctic regions. On a priori grounds 

 too there would seem to be something to be said for a 

 series of circumpolar regions ; as the earth cooled life 

 would be able to advance from the poles towards the 

 equator, and the whole matter has recently been resusci- 

 tated in the polar theory of the origin of faunas (see 

 below). 



