ECOLOGICAL FACTORS 379 



camel is injured by a vapor pressure of more than 11-12 mm. and soon 

 succumbs. 3 Hygric animals are most often stenohygric; xeric animals 

 are much less so. Between the two extremes a median group of so- 

 called mesic animals can be recognized which may be euryhygric or 

 stenohygric. These thrive best under moderate conditions and may be 

 killed by extended exposure to dry or moist extremes. 



High humidity excludes animals adapted to arid conditions in many 

 fewer instances than does the reverse condition. Gadow deduces from 

 the study of the distribution of reptiles in Central America that it is 

 easier for the xeric northern genera and even species to extend their 

 ranges to the south, and to adapt themselves to a more uniformly 

 warm and decidedly damp region with a luxuriant vegetation, than for 

 the hygric southern animals to range to the north. 4 Regions with 

 uniformly warmer and damper air, like the Amazon region, the rain- 

 forests of the Congo, or the Island of Ceylon, possess a rich plant life 

 and can, therefore, support an abundance of animals. Such regions, 

 like islands and coastal regions, are especially suitable for animals 

 adapted to humid conditions; the land planarians thrive on the 

 Malayan islands; the West Indian Islands possess easily as many 

 species of land snails as all the rest of America from Alaska to Tierra 

 del Fuego; and the Philippines are richer in terrestrial snails than 

 the Indian and Indo-Chinese peninsulas together. 5 



Animal life of the arid regions is sparse, corresponding to the 

 scanty development of the plant life and the severity of selection. 

 In dry regions the snails have unusually thick shells, as in Bulimus 

 detritus, or they protect themselves by a thick mucus, as in Arion 

 empiricorum. Clausilian snails, abundant in hot, dry, calcareous moun- 

 tains, close the mouth of their shell by means of an operculum; some 

 species, however, that inhabit the seacoasts or misty altitudes, have 

 a weaker operculum or none at all; Alopia maxima, for example, has 

 no operculum on the peaks of the Muguria Mountains in Transylvania ; 

 200 m. lower the same species occurs with a well-developed operculum. 

 The isopod, Porcellio scaber, ranges in Germany to the limit of the 

 coast climate, with its high humidity, and also occurs abundantly on 

 dry, sandy soils; in the interior, however, it is found only in damp 

 places, under stones and underbrush or in cellars. 7 



The humidity of the air seems also to have an effect on the colora- 

 tion of animals, in that darker colors tend to appear with greater 

 humidity; but this is not applicable to all animal species nor to all 

 degrees of humidity. Crickets, Gryllus campestris, reared in a moist 

 environment with a relative humidity of 60-80%, produced deep black 



