234 
ORIGIN OF LIFE IN AMERICA 
arrived at are almost as sure as if we had geological proof 
of these successive subsidences. Tlie author of the “Geo¬ 
graphical Distribution of Animals ” thus not only claims that 
zoogeography is of great value as an interpreter of geological 
phenomena, but he feels confident of having indicated the 
correct solution to the problem of the faunistic relationship 
between the two continents. There are one or two points, 
however, in the fauna of South and Central America, which 
seem to throw doubt on Dr. Wallace’s interpretation of the 
facts of distribution. He states (p. 58) that when the final 
union of the two continents took place, the tropical climate 
of the lower portion of Guatemala and Mexico would have 
invited rapid immigration from the south, while some 
northern forms would have extended their range into and 
beyond the newly elevated territory. That the fauna of South 
America has advanced across the isthmus, and has even in¬ 
vaded the State of Texas, appears probable from a study of 
the existing fauna. We need not even cite the presence of 
the South American fossils in North America in favour of 
this view. All sections, indeed, of the fauna seem to have 
taken part in this northward advance, even the fishes. 
Take for example the members of the family Cichlidae. 
Over one hundred and fifty species of this strictly fresh¬ 
water group are known from America, the great majority 
being confined to South America. Mr. Regan * is of opinion 
that the Central American and Mexican species are more 
specialized than the South American ones, and that the 
former have certainly been derived from the latter. No 
member of this family of fishes is known from the Mexican 
plateau, and only three or four species extend northward 
on either side of it, North America being almost devoid 
of Cichlidae (see Fig. 12). All the available evidence 
thus points to South America as the place of origin of the 
family, as far as the New World is concerned ; and thence it 
has spread northward, like hosts of other groups of animals. 
This advance must, of course, have taken place as soon as a 
connected land passage with suitable lakes and rivers enabled 
them to proceed northward. Dr. Wallace thinks that the 
sudden appearance of the large South American edentates 
* Regan, 0. Tate, “Fishes of Central America,” pp. xiii—xvi. 
