264 GEOGRAPHICAL AKD 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 contrury direction. 

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

 the abyssal fauna of the Gulf of Mexico only ten per cent, arc miv.h 

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

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

 the cold bottom wastes was effected by a descent from the [lolar 

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

 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 moUusk fauna," that in all probability "the 

 abyssal regions have local faunte 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" MoUusca as 

 compared with those of the "Blake." 



The hypsometrical 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, tod pomegranate 

 (450 to 2,100 feet); 2. That of the oak and pine (2,100 to 8,600 

 feet); and, 8. That of the cedar and green turf (8,600 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 

 18,000 feet, while from the Himalayas Anadoniis Schlagintweiti 

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

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

 tives of extreme hypsometric range appear to be Pupa alticola and 



