184 



ANALYSIS OF THE ENVIRONMENT 



tained at about 35° or 36° C; at 40°, twice 

 as much heat is dissipated as is produced. 

 There are numerous contrasts in heat regu- 

 lation between the sweating and nonsweat- 

 ing mammals. Among other diflFerences, the 

 sweating species increase their pulse rate as 

 they become hot; their blood is routed to 

 the cooled surface layers, and the respira- 

 tion rate may decrease. In contrast, non- 

 sweating animals show a decreased pulse 

 rate when hot; the blood is routed away 

 from the hot surface, and the rate of respi- 

 ration increases to the well-known heat 

 panting. Both types avoid long exposure to 

 the hot sun. 



Acclimatization to heat is striking and 

 important, both in direct connection with 

 efficiency in living and as an illustration of 

 the general principle of acclimatization. In 

 becoming accustomed to the desert, high 

 altitudes, or other environmental extremes, 

 a warm-blooded animal shows physioloe;i- 

 cal changes that promote maintenance, for 

 example, of suitable body temperature, of 

 salt, and of water content. Acclimatization 

 to changed conditions seems to be an ex- 

 pression of a latent capacity that develops 

 under appropriate stimulation. The range of 

 environments is so great that even a euryo- 

 kous organism, such as man, cannot be 

 ready at any one time to cope with all to 

 the optimum extent. 



Different degrees of water saving are at 

 the basis of an ecological classification com- 

 parable in importance to that based on 

 tolerance of salinity among aquatic forms 

 or on temperature relations for all organ- 

 isms. Plants that 2;row only in water or wet 

 places, such as swamps or wet meadows, 

 are called hydrophtftes. Plants of forests or 

 prairies that grow in regions where there is 

 neither an excess nor a deficiency of water 

 are TnesovhiitP<!. Those that live in dry situa- 

 tions subjected to hi8;h evaporation stresses 

 are xerophiftes (Weaver and Clements, 

 1929).* Animals with similar habitat rela- 

 tions are hydrncoles if they live in water 

 {hygrocolea if living in moist places^, meso- 

 coles, and xerocoles. respectively. Both 

 plants and animals develop certain struc- 

 tures related to the amount of moisture they 

 normally encounter and so show hydromor- 

 phic, mesomorphic, and xeromorphic fea- 



• The suffix phijte refers definitely to plants: 

 hence it is not appropriate to sneak of xero- 

 phytic animals or even of xerophytic habitats. 



tures. Differences in physiology and in 

 habits also allow animals to adjust their 

 water requirements to the available supply. 



Animals obtain water (a) by drinking, 

 (b) by absorbing it through their skin from 

 contact with some damp object, as toads or 

 frogs get water from damp ground (Adolph, 

 1932), (c) directly from their food, or (d) 

 from water produced by metabohsm, as do 

 most terrestrial insects that feed on dry 

 food materials. The method of securing 

 water and the relation to the supply of 

 liquid water, as well as resistance to the 

 drying effects of the surrounding atmos- 

 phere, are important in determining the 

 distribution of animals. It may be doubted 

 whether animals absorb water from an at- 

 mosphere saturated with water vapor 

 except under special conditions when the 

 vapor tension of the surrounding air is 

 greater than that of the water-permeable 

 surface of the animal. 



A consideration of the precedinej discus- 

 sion about the vapor tension, together with 

 Adolph's observations on the water rela- 

 tions of frogs (1932, 1933), shows the 

 reason for this inability. When Adolph ex- 

 posed frogs to 



"... Saturated atmospheres under rigidly 

 unifonn temperatures it was found that evapo- 

 ration still went on. Hence, under no steady 

 conditions could a frog ?ain water from the 

 atmosphere. Tlie reason for this is one that 

 holds for all organisms and tissues; it is that 

 the fros; is continually producins; heat, thus 

 raisincj its temperature above that of its sur- 

 roimdings, hence enabling it to evaporate water 

 bv raising the dew-point of the air in contact 

 with its surface." 



Contrariwise, Ludwig (1937) holds that 

 grasshoppers are hygroscopic and can ab- 

 sorb water from air with a high moisture 

 content. The difference, if real, may be re- 

 lated to the relative impermeability of the 

 grasshopper's exoskeleton. 



The adjustments that permit animals to 

 live surrounded by a drying atmosphere in- 

 clude, among others, the following adaptive 

 features: 



1. A more or less impervious integument 



2. Internal lungs or tracheal system 



3. Water saving: 



(a) By the secretion of concentrated 

 and even of crystalline nitrogenous 

 waste 



(h) By depositing dry feces 



4. Suspended animation 



