LIMITA TION B Y BARRIERS. 1 8/ 



is accounted for, some species migrating south as 

 well as north in search after a cooler climate. Wal- 

 lace has very carefully followed out this explanation 

 according to the accepted principles of geology, and 

 finds it a happy explanation for great masses of 

 peculiar facts. 



But, with all of these suggestions, we must not 

 imagine that the difficulties are removed ; many 

 facts still remain for which no sufficient explanation 

 has been given. Most of them are isolated cases, 

 each of which must demand its own separate expla- 

 nation, and they are certainly becoming fewer in 

 numbers as we become more acquainted with the 

 present and past history of animals. Probably the 

 greatest difficulties which the present distribution 

 of animals offers to the descent theory, concern the 

 wide disf>ers*al of fresh-water organisms. Even 

 when we have granted all that is claimed as to the 

 occasional means of dispersal by changes in the 

 level of the land, by floods, by transportation of eggs, 

 by whirlwinds, by floating ice, etc., even when we 

 have granted that these means have accomplished 

 much, it still remains a marvellous fact that a cer- 

 tain fresh-water fish, Galaxias attenuatus, is found 

 in the rivers of Tasmania, New Zealand, the Falk- 

 land Isles, and the mainland of South America. 

 How to explain such a case as this, it is difficult to 

 say, unless it be regarded as a case where a former 

 species of almost universal distribution has become 

 extinct except in these localities ; and this explana- 

 tion is any thing but a happy one. A similar, 

 though less striking, puzzle is offered by the occur- 



