378 LAND ANIMALS 



water surfaces, upon rainfall, and upon the nearness of mountains, 

 which cause a condensation of water vapor. Sea winds bring damp air; 

 continental winds are dry. The higher the moisture content of the air, 

 other things being equal, the more suitable it is for the development 

 of animal life; it requires fewer adaptations on the part of primitive 

 air-inhabitants which are descended from aquatic ancestors. 



The separation of animals into those adapted to moist conditions 

 and those to drought (hygric* and xeric animals) has been mentioned 

 (p. 49). The former include all air-inhabiting animals with insufficient 

 protection against drought, such as land planarians, terrestrial leeches, 

 earthworms, land isopods, snails, and amphibians; xeric air-dwelling 

 animals, adapted to aridity, are specially protected against desiccation, 

 as, for example, most of the insects, spiders, reptiles, birds, and mam- 

 mals. Nevertheless, the above examples are not without exception. 

 Desert isopods, derived from the generally hygric land isopods, are 

 found in arid regions, and many snails live exclusively on dry southern 

 slopes, as Bulimus detritus and the species of Xerophila. Many reptiles, 

 especially the lizards, give off remarkably little water vapor, while 

 various species of crocodiles and Python molurus vaporize an abundant 

 amount. Indeed, though cattle in general give up much water and are 

 therefore hygric, there are strains in the southern part of the arid 

 Lower Californian peninsula that drink only every two days or even 

 only once every week. 2 



We can carry the above classification still farther, in that we may 

 distinguish between the euryhygric and stenohygric animals, those that 

 endure more or less variation and those that are harmed by the 

 greater fluctuations of humidity. To the euryhygric group belong many 

 insects, such as cerambycid beetles, flies, dragonflies, and many birds 

 and mammals. The roe deer, Cervus capreolus, for example, occurs in 

 the swampy lowlands of the Danube as well as in the dry valleys of 

 the Swabian Alps. One can also designate as euryhygric the terrestrial 

 leeches, Xerobdella lecomtei, or the genus Armadillidium, of the land 

 isopods, or the common toad, that can live in regions of low humidity 

 without being limited to them. Stenohygric animals again can be hygric 

 or xeric. Land planarians, many snails (Succinea) , may flies and 

 mosquitoes, the buffalo (Bubalus) , and the hippopotamus can endure 

 only slight reduction in humidity; on the contrary, other snails, like 

 Bulimus detritus or Xerophila, can live only in dry regions, and the 



* "Hygric" is here preferred to "hydric" in order to restrict the latter term to 

 amphibious animals. 



