ZOO-GEOGRAPHICAL REGIONS. 
399 
Himalayas and liy the southern waterslied of the Yang-tse-Kiang, 
meeting with the Pacific near Shanghai ; the west boundary is formed 
hy the desert lands of North-West India and South Central Asia. 
This region is distinguished malacologically hy the great develop- 
ment of Operculates and the predominance of the Epiphallogonous 
gToup of Helices, peculiarities shared with the West Indian Islands. 
The Ethiopian region includes Southern Arabia, all Africa south of 
the Sahara, and the Malagasian sub-region. Its fauna is of a very 
lowly stamp, especially in its extreme southern part and in the sub- 
sidiaiy island of Madagascar, the charactei’istic mollusks showing a 
remarkable affinity with those of Australia and other primitive regions. 
Tlie Arctic region is circumpolar and extends from the Arctic 
ocean on the north to the limits of the Nearctic and Palmarctic 
regions respectively, and although possessing so limited a fauna, it 
cannot well be treated otherwise than as a distinct region, as the 
Pahnarctic and Nearctic regions have each a Helicidian fauna of a 
radically different stamp, the Euadenia being dominant in the latter 
region, while in the former they are decidedly a waning race. 
Although the fauna of this region is restricted chiefly to freshwater 
races and to Helices and other genera of small size, capable of resisting 
a rigorous climate, possibly diffused throughout the Canadian region of 
North America by migration from Northern Europe, over the assumed 
ancient Pliocene connection by way of Greenland ; the fauna is yet in 
a measure comparable with the Mongolian fauna, as the region is 
occupied in great part by the weaker terrestrial species thrust aside 
hy the tide of stronger forms advancing from the European sub-region 
across Central Asia along the more fertile tract of country north of 
the desert region. 
The freshwater .species, however, inhabiting the extensive and con- 
nected lake region of boreal North America, will, from the keener 
competition they necessarily contend against, tend naturally to 
become more dominant than individuals, even of the same species, 
confined to more restricted and less competitive areas. 
The Nearctic region embraces the continent of North America, 
south of the Arctic region, and may be divided into three sub-regions : 
Californian, Central and Alleghanian, but owing to the insuperable 
obstacle to the ready migration of the stronger races presented by the 
mountain ranges which bisect the continent from north to south there 
is a great inequality in the vigour of the occupants of the different 
