BUKEOWING LIZAEDS 203 



two southern corners of North America, one (Ehineura) 

 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 (Glaucoma dulcis) lives in 

 Mexico, New Mexico and Texas, the other (Glaucoma 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 



