222 ORIGIN OF LIFE IN AMERICA 



Dr. Ruthven's valuable researches into the origin and dis- 

 tribution of the garter-snakes (Thamnophis = Eutenia) have 

 been discussed in an earlier chapter (p. 128). I need only 

 restate that he traces the home of the genus to northern 

 Mexico. And Thamnophis is clearly an offshoot from the 

 older water-snakes (Tropidonotus), which have almost a 

 world-wide range. As in the case of Potamobius and 

 Cambarus, both genera seem to have spread northward 

 from their south-western centre, the south-western Tropi- 

 donotus validus having its nearest relation in the Sar- 

 dinian Tropidonotus viperinus. Dr. Brown * recognised per- 

 fectly that the affinity between such forms as the European 

 and American species of Tropidonotus necessitated the exist- 

 ence of a former land bridge between the two continents. 

 He also urged that the existence of this bridge must ha,ve 

 coincided with a warm climate in the north, for he naturally 

 assumed that only in the extreme north could there have been 

 such a land connection. Its geological age he fixes at about 

 the early Miocene, though he believes many of the present 

 genera to have been in existence even in Eocene times. 

 My objection to Dr. Brown's theory is that we nave no evi- 

 dence in Europe of a southward advance of Tropidonotus 

 from a former northern centre of distribution, nor are the 

 northern species in both continents more clos-ely related to 

 one another than the southern species. The former existence 

 of a more southern trans -Atlantic land bridge in early 

 Tertiary times, on the other hand, is supported by such a 

 number of palaeontological facts, as we shall learn later on, 

 that the evidence is overwhelmingly in its favour. 



It is quite possible that the western tortoise Clemmys 

 marmorata, or its ancestors, for the genus, as I mentioned 

 (p. 133), has inhabited south-western North America since 

 Eocene times, has spread across the same mid-Atlantic land 

 bridge to western Europe, a near relation (Clemmys leprosa) 

 being peculiar to Portugal, Spain and north-western Africa. 



That all these animals living in south-western North 

 America and western Europe which show close relationship, 

 are relicts of very remote geological times is rendered prob- 



* Brown, A. E., " Post-Glacial Nearctic Centres," p. 466. 



