232 
ORIGIN OF 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 fauna 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, for 
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 — 784.” 
