138 ORIGIN OF LIFE IN AMERICA 



Their earliest centre of distribution lay in what has been 

 called Sonoraland, or south-western North America, and 

 from here the species have spread in Miocene times, according 

 to Dr. Gadow, to the eastern States, to the island of Haiti 

 and even to Peru. The latter occurrence is of particular 

 importance, as we shall see later on, when we come to the 

 consideration of the points of resemblance between the 

 Mexican and the South American faunas. Still more remark- 

 able is the fact that a single species of Spelerpes (S. fuscus) is 

 known from some of the remaining fragments of the ancient 

 Tyrrhenian continent in southern Europe (see Fig. 8). Even 

 Dr. Gadow,* who shows little inclination for reconstructing 

 ancient land bridges, does not suggest that this salamander 

 could have crossed the Atlantic Ocean without their 

 assistance. He thinks a land connection joining north- 

 eastern North America with north-western Europe, by way 

 of Greenland, might have enabled the south European 

 Spelerpes to cross from the New World to the Old. 

 I concur with Dr. Gadow in the belief of the former 

 existence of a land bridge in the extreme North Atlantic, 

 but I am of the opinion that it had not yet made its appear- 

 ance at the time when Spelerpes undertook its journey to 

 Europe. 



This short review of some of the characteristic north- 

 eastern reptiles and amphibians has clearly revealed a 

 relationship of some of the older forms with those of southern 

 and eastern Asia and also of southern Europe. In several 

 instances it was demonstrated that the eastern States were 

 not the original home of the genera, but that the North 

 American centre of distribution lay in the south-west. Hence 

 it seems possible that the south-west was in remote times, say 

 about the commencement of the Tertiary Era, the great centre 

 from which reptiles and amphibians wandered eastward. 

 Owing to subsequent changes in the climatic conditions of 

 the south-west, some genera, and even families, probably 

 became extinct there, thus obscuring the original relation- 

 ship of that part of North America with Asia and Europe. 

 Without fossil evidence to guide us, it would seem as if these 



* Gadow, H., " Mexican Amphibians and Keptiies," p. 244, 



