ORIGIN OF THE GARTER SNAKES 129 



from north Australia through the Malay Archipelago and 

 northward to Japan. Westward it extends to India, Africa 

 and Europe. In America the genus occurs principally 

 in the eastern States, but there is also a species in Cuba 

 and several in Central America. None have penetrated 

 to South America. From Dr. Boulenger's * catalogue and 

 Dr. Stejneger's description of Japanese forms,f it seems 

 evident that the east Asiatic forms are not very closely 

 related to the American ones. On the other hand, Tro- 

 pidonotus validus, from Lower California, Arizona and 

 Mexico, is nearly akin to the west European Tropidonotus 

 viperinus, whose high antiquity is indicated by its occur- 

 rence in Sardinia and Algeria. I may have another oppor- 

 tunity later on of dealing with this interesting genus, but 

 it must be conceded that although we possess no fossil 

 evidence, the origin of Tropidonotus must date back at least 

 to the commencement of the Tertiary Era. In another chapter 

 I shall show how extraordinarily intimate is the relationship 

 of the south-west American and the West European faunas. 

 Evidence will then be given in support of the theory that 

 southern Europe and the south-western parts of North 

 America have been connected with one another by land, and 

 that we probably owe the resemblance in the animals and 

 plants of these two regions to that fact. The species of 

 Tropidonotus, in America at any rate, are semi;-aquatic. They 

 are so much attached to water that they are popularly known 

 as " water-snakes," whereas the garter-snakes are much less 

 bound to the neighbourhood of water. The latter are not 

 uncommonly found on higher ground and in drier situations, 

 a change in the climatic conditions of the south-west may thus 

 originally have given rise to the Thamnophis branch from the 

 original Tropidonotus stock. The headquarters of Tropi- 

 donotus now lie in the eastern States, where the conditions 

 for its existence are more favourable than in the south-west. 

 All the same, it seems probable that the original centre of dis- 

 persal was in the south-west, for when Tropidonotus arrived 



* Boulenger, GK A., " Catalogue of the Snakes in the British Museum,', 

 Vol. 1. 

 . f Stejneger, L., " Herpetology of Japan," pp. 264 294. 



L.A. K 



