234 ORIGIN OF LIFE IN AMERICA 



arrived at are almost as sure as if we had geological proof 

 of these successive subsidences. The 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 fax 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, C. Tate, " Fishes of Central America," pp. xiii- xvi. 



