WILD LIFE OF ORCHARD AND FIELD 



size, number of species in a given district, and in- 

 tensity of color decreasing as you go away from 

 the equator; but this statement must be taken in 

 a very general sense. Different quarters of the 

 globe are characterized by special groups of land 

 mollusks as of other animals thus, Achatinella, 

 with three hundred species, is confined to the Sand- 

 wich Islands. But Helix the true snail with its 

 many subgenera and two thousand species, is ab- 

 solutely cosmopolitan. The fresh -water forms, 

 also, are spread everywhere, except in Australia, 

 and flourish in cold countries, Pupa having the 

 hardihood to live nearer the north pole than any 

 other known shell. Yet it is a remarkable fact 

 that, however erratic and extensive may be the 

 range of the genera to which they belong, the ma- 

 jority of the species of pulmonates of all sorts have 

 an extremely limited habitat, in some cases com- 

 prising only a few square rods ; and here they seem 

 to have a certain favorite resting-place, or "home/' 

 to which they return after feeding excursions. A 

 second noteworthy fact, obtaining in no other ex- 

 tensive group of animals, is that many more species 

 of land shells exist in the islands than on the con- 

 tinents of the world. Mr. A. R. Wallace accounts 

 for this curious fact by explaining how certain 



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