GEOGRAPHICAL DISTRIBUTION 331 



Gnathodon), then the arctic and temperate productions 

 will at a very late period have marched a little further 

 north, and subsequently have retreated to their present 

 homes ; but I have met with no satisfactory evidence 

 with respect to this intercalated slightly warmer period, 

 since the Glacial period. 



The arctic forms, during their loner southern migra- 

 tion and re-migration northward, will have been ex- 

 posed to nearly the same climate, and, as is especially 

 to be noticed, they will have kept in a body together ; 

 consequently their mutual relations will not have been 

 much disturbed, and, in accordance with the principles 

 inculcated in this volume, they will not have been 

 liable to much modification. But with our Alpine pro- 

 ductions, left isolated from the moment of the return- 

 ing warmth, first at the bases and ultimately on the 

 summits of the mountains, the case will have been 

 somewhat different ; for it is not likely that all the 

 same arctic species will have been left on mountain- 

 ranges distant from each other, and have survived there 

 ever since ; they will, also, in all probability have be- 

 come mingled with ancient Alpine species, which must 

 have existed on the mountains before the commence- 

 ment of the Glacial epoch, and which during its coldest 

 period will have been temporarily driven down to the 

 plains ; they will, also, have been exposed to somewhat 

 different climatal influences. Their mutual relations 

 will thus have been in some degree disturbed ; conse- 

 quently they will have been liable to modification ; 

 and this we find has been the case ; for if we compare 

 the present Alpine plants and animals of the several 

 great European mountain-ranges, though very many 

 of the species are identically the same, some present 

 varieties, some are ranked as doubtful forms, and some 

 few are distinct yet closely allied or representative 

 species. 



In illustrating what, as I believe, actually took place 

 during the Glacial period, I assumed that at its com- 

 mencement the arctic productions were as uniform 

 round the polar regions as they are at the present day. 



