264 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



distribution of many of the species has led to the belief that such 

 is the case, but, on the other hand, certain facts that have recently 

 been brought to light seem to point in the contrary direction. 

 Thus, it has been shown by Mr. Dall that of the species composing 

 the abyssal fauna of the Gulf of Mexico only ten per cent, are such 

 as may be termed boreal, a very small proportion to what might 

 have been expected, were it to be assumed that the peopling of 

 the cold bottom wastes was effected by a descent from the polar 

 regions. On the other hand, thirteen per cent, were found to be 

 tropical, and seventy-five per cent, uncharacteristic, forms. It is 

 concluded from these facts, and from the circumstance that "the 

 tropical forms belong to the same groups as those characteristic of 

 the local littoral mollusk fauna," that in all probability "the 

 abyssal regions have local faunae proper to their various portions, 

 and that a universal exclusive abyssal fauna, so far as mollusks are 

 concerned, does not exist." This conclusion, which was concurred 

 in by the late Mr. Gwyn Jeffreys, receives further support, it is 

 claimed, from the distinctness of the ' ' Challenger " Mollusca as 

 compared with those of the " Blake." 



The hypsornetrical distribution of the Mollusca is governed 

 almost exclusively by conditions of climate and food-supply, the 

 influence of the latter being manifest in the intimate relation 

 which binds many of the species to the plants upon which they 

 habitually feed. Thus, in the Higher Kabylia, Aucapitaine has 

 framed three molluscan zones, each corresponding to a particular 

 plant growth : 1. The zone of the ash, olive, and pomegranate 

 (450 to 2,100 feet); 2. That of the oak and pine (2,100 to 3,600 

 feet); and, 3. That of the cedar and green turf (3,600 to 7,200 

 feet). The upper limit to which mollusks attain on the continent 

 of Europe (Alps) is about eight thousand feet, somewhat below 

 the line of perpetual snow ; along the region of the equatorial 

 Andes and the Himalayas the line is placed at about twice this 

 height, also approximating the snow-level. Five species of fresh- 

 water shells, of the genera Planorbis, Paludestrina, and Cyclas, were 

 found by Morelet to inhabit Lake Titicaca at an elevation of nearly 

 13,000 feet, while from the Himalayas Anadenus Schlagintweiti 

 has been obtained at a height of 16,500 feet, and Limnsea Hookeri 

 at 18,000 feet. Of the North American land shells the representa- 

 tives of extreme hypsometric range appear to be Pupa alticola and 



