164 THE GENESIS OF SPECIES. [CHAP. 



certainly a wonderful fact that New Zealand should have 

 a closer resemblance in its Crustacea to Great Britain, its 

 antipode, than to any other part of the world : " and Mr. 

 Darwin adds : " Sir J. Richardson also speaks of the reap- 

 pearance on the shores of New Zealand, Tasmania, etc., of 

 northern forms of fish. Dr. Hooker informs me that 

 twenty-five species of algae are common to New Zealand 

 and to Europe, but have not been found in the intermedi- 

 ate tropical seas." 



Many more examples of the kind could easily be 

 brought, but these must suffice. As to the last-mentioned 

 cases, Mr. Darwin explains them by the influence of the 

 glacial epoch, which he would extend actually across the 

 equator, and thus account, among other things, for the 

 appearance in Chili of frogs having close genetic relations 

 with European forms. But it is difficult to understand the 

 persistence and preservation of such exceptional forms with 

 the extirpation of all the others which probably accom- 

 panied them, if so great a migration of northern kinds had 

 been occasioned bv the glacial epoch. 



Mr. Darwin candidly says, 17 " I am far from supposing 

 that all difficulties in regard to the distribution and affini- 

 ties of the identical and allied species, which now live so 

 widely separated in the North and South, and sometimes 



on the intermediate mountain-ranges, are removed." 



" We cannot say why certain species and not others have 

 migrated ; why certain species have been modified and 

 have given rise to new forms, while others have remained 

 unaltered." Again he adds : " Various difficulties also re- 

 main to be solved ; for instance, the occurrence, as shown 

 by Dr. Hooker, of the same plants at points so enormously 

 remote as Kerguelen Land, New Zealand, and Fuegia ; but 

 icebergs, as suggested by Lyell, may have been concerned 

 in their dispersal. The existence, at these and other dis- 

 17 "Origin of Species," 5th edit., p. 459 



