Geographical Distribution. 219 
faunas range northward and southward in parallel 
lines not far from each other, under corresponding 
climates” : they are, however, “separated from each 
other by impassable barriers, either of land or open 
sea”: and it is in exact coincidence with the course of 
these barriers that we find so remarkable a differen- 
tiation of the faunas’. Obviously, therefore, it is 
impossible to suggest that this correlation is accidental. 
Altogether many thousands of species are involved, 
and within this comparatively limited area they are 
sharply marked off into three groups as to their 
natural affinities, and into three groups as to their 
several basins. Hence, if all these species were 
separately created, there is no escape from the con- 
clusion that for some reason or another the act of 
creation was governed by the presence of these 
barriers, so that species deposited on the Eastern 
shores of South America were formed with one set of 
natural affinities, while species deposited on the 
Western shore were formed with another set; and 
similarly with regard to the third set of species in the 
third basin, which, extending over a whole hemisphere 
to the coast of Africa without any further barrier, 
nowhere prcsents, over this vast area, any other case 
of a distinct marine fauna. But what conceivable 
reason can there have been thus to consult these 
geographical barriers in the original creation of specific 
The only exception is in the case of the fish on each side of the 
Isthmus of Panama, where about 30 per cent. of the species are identi- 
cal. But it is possible cnouzh that at some previous time this narrow 
isthmus may have been even narrower than at present, 1f not actually 
open. At all events, the fact that this partial exception occurs just 
where the land-barrier is so narrow, is more suggestive of migration 
than of independent creation. 
