2 2 o Quat terly Jottmal of Conchology. 



come under my observation, as "San Juan," and there are perhaps 

 as many more "San Juan's" south of that specially quoted herein 

 on the westerly coast of America, in the Central and South Amer- 

 ican States, 



Attention is directed to the fact that the three species herein 

 mentioned as exhibiting extraordinary vitality, belong to geograph- 

 ical areas, which receive only minimum rainfall, or which are, in 

 simple language, nearly rainless regions. 



Within such areas, vegetation is extremely limited even in favour- 

 able seasons, and the presence and growth of the annual plants is, 

 of course, dependent on the rainfall; this last occurring infrequent- 

 ly makes the food supply of land moUusks and other phytophagous 

 or vegetable-eating animals exceedingly precarious. 



It is highly probable that a careful investigation in this direction 

 will lead us to the conclusion, that the land mollusks which inhabit 

 arid areas have, through selection, adaptation and evolution, be- 

 come especially fitted for the contingencies of their habitat, and 

 possess a greater degree of vitality or ability to live without food 

 than related forms in what may be considered more favorable 

 regions, and through and by reason of their long sleep or hiberna- 

 tion, more properly estivation, with its inactivity and consequent 

 immunity from any waste or exhaustion of vital strength, are en- 

 abled to maintain their hold upon life when animals more highly 

 organised would inevitably perish; and we are furnished with an 

 illustration, in the instances cited, how nature works compensa- 

 tively, when we institute a comparison with the opposite condition 

 of activity, and the food required to sustain it. 



