130 ORIGIN OF LIFE IN AMERICA 



in North America the climate of that region must have been 

 very different from what it is at present. 



I may just mention two other examples of innocuous snakes 

 which frequent the north-eastern States, viz., the smooth 

 green snake (Liopeltis vernalis), and the rough green snake 

 (Cyclophis aestivus). The former is abundant in New York 

 State and northward as far as south-eastern Canada. South- 

 ward it ranges to the Gulf of Mexico and westward to New 

 Mexico, becoming rarer as we approach the drier and warmer 

 districts. The other does not extend nearly so far north. On 

 the other hand, the rough green snake is found westward as 

 far as northern Mexico and California. Both of them share 

 the peculiarity of being the only members known in America 

 of the genera to which they belong. That is not the only fea- 

 ture of interest about their distribution. I have just urged 

 that the ancestors of the American species of Tropidonotus 

 must have come from Europe. We cannot claim the same 

 origin for the American species of Liopeltis and Cyclophis, 

 for neither of these genera inhabits Europe. Both of them are 

 absent also from Africa. Their headquarters are in southern 

 and eastern Asia, but they do not extend as far north as Japan. 

 Formerly these snakes were classed among that insoluble 

 zoogeographical enigma, namely, the group of animals and 

 plants peculiar to eastern Asia and eastern America. Now 

 we have advanced in so far as we have been able to trace 

 some of the eastern forms to an originally western American 

 range. It has been made easier, therefore, for those natura- 

 lists who are in the habit of explaining anomalies of dis- 

 tribution by the convenient flotsam -jetsam theory, to bring 

 their views to bear upon problems such as those suggested 

 by the two green snakes. That these snakes could have been 

 floated across the Pacific Ocean on a raft by any possible 

 chance, is to me inconceivable. That they should have utilised 

 the Bering Strait land connection, and subsequently have 

 become extinct all along north-eastern Asia and north-western 

 North America does not appeal to me either as likely. We 

 must only leave the consideration of the problem for the pre- 

 sent, as was done in the case of the lizard genera Eumeces 

 and Lygosoma, which also apparently had an east Asiatic 

 origin. 



