4 THE NATURAL HISTORY OF CHINA 



tion, and the living grizzlies of North America. The only 

 way in which the latter could have acquired their present 

 distribution was by the migration or, perhaps it would be- 

 better to say, the gradual spread, of their ancestors from 

 Europe across Siberia or Central Asia to the American con- 

 tinent by way of the land bridge that once existed where 

 the Bering Sea now lies. 



This land bridge was a very important factor in the 

 present distribution of the animals of both America and the 

 Eurasian land mass. By its means such animals as the 

 camel and the horse, both of which first developed in North 

 America, and subsequently became extinct in that continent, 

 arrived in Asia, the horse passing on to Europe, where it 

 became the servant of man, and was subsequently re- 

 introduced into America by him. 



"When we come to examine the distribution of the cold 

 blooded vertebrates, such as reptiles and fishes, we have to 

 go> further back in the geological history of the country in 

 order to understand its significance, and it is here that our 

 want of knowledge is most keenly felt. Nevertheless, a few 

 interesting facts may be culled from what we already know, 

 facts which throw a certain amount of light upon the 

 subject. 



An examination of the map of the Old World will reveal 

 the fact that a desert belt stretches from Morocco in North 

 Africa right across Asia to the borders of Manchuria, where 

 it stops within a hundred miles or so of the sea. It has 

 been suggested that it was this desert belt, known to be of 

 considerable age, that prevented the Urodela, or tailed 

 amphibians, from spreading south from Europe and North 

 Asia into Africa and India. Force is given to this conten- 

 tion by the fact that it is only in the extreme eastern part 

 of their range in Eurasia that they occur south of the desert 

 belt in question, for there they found a stretch of humid 

 country by means of which they could spread southward 

 with safety. It is easy to see how such animals as newts 

 and salamanders, which depend entirely upon the presence 

 of ponds, streams, or lakes, wherein to lay their eggs and 

 where their young develop, and which themselves cannot 

 exist in any but a humid environment, would find it utterly 

 impossible to cross a stretch of dry, sandy desert. 



Incidentally it may be mentioned that another animal 

 that appears to have been influenced in its distribution by 

 this desert belt is the roedeer (Capreolus), whose range ex- 

 tends from the extreme west of Europe throughout that 

 continent, Central Asia, north of the deserts, and Siberia 



