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 have 
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 have 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 closely 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. 138), 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. 4GG. 
