232 OEIGIN OP LIFE IN AMERICA 



and Europe were Joined in the south, while northern. Asia was 

 in all likelihood disconnected from the former. The principal 

 point which I think has been clearly demonstrated is that the 

 south-western centre of dispersal has exerted a powerful in- 

 fluence on the development of the living faun.a of North 

 America. There still remains one other feature that I wish 

 to allude to before concluding this chapter. 



I have mentioned above that, while eastern Mexico was 

 either wholly or partially submerged by the sea during later 

 Mesozoic and early Tertiary times, most of western Mexico 

 and a portion of the Californian coast remained dry land, as 

 far as we know, all through geological history. It ought on 

 that account to be a preservation ground for all kinds of 

 relicts of bygone ages. And so it is. I have cited a number 

 of them already, although the country can scarcely be con- 

 sidered as being exhaustively explored. The fresh-water 

 fishes more than any other group show what zoological riches 

 may still be discovered there. The Lerma river system, foB 

 example, in south-western Mexico has a fish fauna, accord- 

 ing to Dr. Meek,* which is quite as distinct and character- 

 istic as if it were on an island in the sea. Of the forty-nine 

 species of fishes now known to occur in this area, not a single 

 one is found elsewhere. These forty-nine species belong to 

 seventeen genera, ten of them being quite peculiar to this 

 region. This result is all the more surprising, as it has often 

 been argued, by advocates of accidental introduction, that the 

 eggs of fishes are apt to adhere to the legs or feathers of water 

 birds, being thus easily transported to other river systems 

 or isolated lakes. The extremely distinct and isolated 

 character of the Lerma river area implies that dispersal of 

 fishes is not affected by such agencies of accidental transport. 

 Fishes only migrate from one river to another when a change 

 of drainage occurs in the head waters, or when the stream' 

 itself shifts its course. 



* Meek, S. E., "Fishes of Mexico," pp. 775— Y84." 



