182 THE INFLUENCE OF INANIMATE SUKROUNDINGS. 



affected. We will now go more fully into a few particularly 

 interesting cases of resistance to extreme dryness of the atmo- 

 sphere, or to perfect saturation of the air with moisture. 



I. The power of resistance to a dry atmosphere. The 

 atmosphere that lies near the surface of the earth is never 

 perfectly dry ; but we usually call it so when it is drier than is 

 good for our health or agreeable to our feelings. This occurs, 

 for instance, with tolerable regularity every summer in Wiiiz- 

 burg ; * many tropical regions are distinguished by a very dry 

 climate ; this is the case with the Sahara, the desert plains of 

 Australia, and the desert coast of South America, where it 

 never or very rarely rains; and even in tropical countries 

 which are justly regarded as having a very damp climate as 

 Java or the Philippines we nevertheless speak of a dry season, 

 and everyone who has lived for any length of time in these 

 islands knows that the dryness of the atmosphere there has a 

 very unpleasant effect, although at least 50 per cent, of moisture 

 is always present in the atmosphere. Hence all animals living 

 in such localities must be able to withstand the desiccating 

 effects of the atmosphere if they are to survive ; and if an 

 originally damp climate were suddenly, or even by gradual 

 change through local disturbance or secular variation, to 

 become a dry one, a great number of species would infallibly 

 fall victims to this change in the conditions of existence. 



Nevertheless animals live even in the driest places on the 

 earth ; many of these, indeed, belong to groups of animals of 

 which the greater number require a veiy high degree of mois- 

 ture in the air to enable them to live. This is the case, for 

 instance, with many land Mollusca. Our common Road- 

 snail and those Snails that creep about on trees require a very 

 considerable amount of moisture in the atmosphere, or they 

 cannot eat, digest, and grow. During the dry summer of the 

 Mediterranean regions, even on islands, as the Balearic Isles, 

 the active life and growth of these creatures is interrupted : 

 they bury themselves deep in the dry earth or between slabs of 



* The place where the Author writes ; but the case is the same, of 

 course, with many places on the Continent. 



