BURROWING LIZARDS 
203 
two southern corners of North America, one (Rhineura) 
in Florida, the other (Euchirotes) in Lower California and 
Mexico. The only North American members of the blind 
snakes (Glauconiidae) are limited in their range to the south¬ 
western States. One of them (Glauconia dulcis) lives in 
Mexico, New Mexico and Texas, the other (Glauconia humilis) 
ranges from California as far as Arizona. These degenerate 
worm-like creatures are entirely subterranean, and feed on 
earthworms and larvae of insects. Hence their distribution 
is of great zoogeographical value. Besides the districts re¬ 
ferred to the family is found in the Lesser Antilles, Central 
and South America, south-western Asia and Africa. That 
these snakes should have passed all through the continent of 
North America and through northern Asia in spreading from 
South America to Africa or vice versa without leaving a trace 
of their former wanderings seems to me very unlikely. 
Yet geographical distribution of that kind is frequently ex¬ 
plained by the supposition of a former Bering Strait land 
bridge offering the only means of land communication between 
the Old World and the New. There being no fossil evidence 
to guide us, we must judge such cases altogether from the 
present distribution, and it appears to me that an ancient 
land bridge across the mid-Atlantic explains the latter more 
satisfactorily than the other hypothesis. 
The theory of the former existence of such a land bridge is 
not built upon a single instance of distribution. I have 
mentioned many others in previous chapters, and I shall 
allude to several in subsequent ones. One other striking 
example may appropriately be mentioned here, viz., the dis¬ 
tribution of the boas (Boidae). These are mostly large and 
active snakes. Nevertheless, they are related to the small 
and slowly-moving blind snakes, because, like them, they pos¬ 
sess rudiments of a hip-bone and hind limbs. Boas inhabit 
all tropical and sub-tropical countries. Only in two dis¬ 
tricts do they pass into temperate climates, viz., in the south¬ 
western States of North America and in south-eastern 
Europe. In these countries are found the two closely-allied 
genera Lichanura and Eryx. One of the North American 
boas (Lichanura trivirgata) is confined to the extreme south 
of Lower California, another to southern California and 
