ENVIRONMENTAL CLASSIFICATION 49 



moths {Tineola), which live in wool and hair. These insects are able 

 to use metabolic 34 water and further conserve their water supply by 

 excreting their nitrogenous wastes in solid rather than in liquid form. 

 Many desert-inhabiting animals may be cryptic and nocturnal in 

 habits, and consequently actually hygric rather than xeric. 



The air-breathing vertebrates are xeric in very different degrees. 

 The Amphibia with their glandular skin protected only by a thin 

 horny layer were characterized above as specifically hygric. They exist 

 in moderately dry regions only by means of special provisions. The 

 three remaining classes, reptiles, birds, and mammals, are also diverse 

 in this respect. Reptiles and birds are in general much better designed 

 to withstand dryness than are most mammals. They may be contrasted 

 as water-savers and water-spenders. Reptiles and birds have an advan- 

 tage over mammals in their complete lack of skin glands, which pro- 

 mote evaporation. Besides this they save much water because their 

 excreta are not evacuated in a dissolved state, but as crystalline uric 

 acid; the solution waters are reabsorbed in the kidneys. The ostrich is 

 an exception, excreting fluid urea. 35 



The mammals, by contrast, give off a good deal of water, both 

 through their skin glands and on account of the fact that their excre- 

 ment also contains more water than that of reptiles and birds. There 

 are great individual differences. The skin glands are very unequally 

 developed in the various mammals. When they are present in abun- 

 dance, as in men, apes, horses, and cattle, the loss of water is natu- 

 rally larger than in forms in which they are nearly or entirely absent, 

 as in most rodents and certain ruminants. Between these extremes are 

 such forms as the hedgehog and squirrel, with skin glands sparingly 

 scattered over the body, and the carnivores, for the most part with a 

 small number of skin glands. Man gives off more than 1.5 liters of 

 water per day through his skin by insensible evaporation, i.e., without 

 noticeable perspiration. When the skin glands are transformed into 

 genuine sweat glands, 36 as in man, ape, and horse, and thus take part 

 in the regulation of the body temperature by giving off large amounts 

 of water, the losses of water are notably larger. In dry seasons, a 

 man requires 5 to 8 liters of water daily; at high temperatures and 

 in dry air, the lack of water for more than 24 hours may endanger his 

 life. On the other hand, many mammals can hold out for months 

 without drinking, getting along with the water taken with their food, 

 and with water of metabolism 34 as do mice and porcupines, hyraces, 

 aardvarks, many antelopes and gazelles. 



Evaporation through the lungs, concentration of the urine, and 

 water content of the excrement are important factors in water econ- 



