52 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



which, however, must become dissolved to be perceived. The sense of 

 smell acquires a great importance in some air-breathers (many insects 

 and most mammals), for general orientation, and may even replace 

 vision in importance. It is especially important to nocturnal and cave 

 animals. This importance of smell depends in part on the tendency 

 toward uniform diffusion of gases and the resulting wide and uniform 

 distribution of odorous substances in the air. As the organs of taste 

 and smell must have cells of living protoplasm at the surface, they 

 are especially subject to the danger of drying. In the air-breathing 

 vertebrates they are protected by being placed in special cavities or 

 chambers, and are there kept moist by means of special glands. In 

 aquatic animals, even in fishes, the organs of chemical sense may be 

 distributed over the whole surface. 



Organs of hearing are enormously more developed among terrestrial 

 animals. These are found in the terrestrial vertebrates, in the insects, 

 and perhaps among the arachnids. They are wanting in primarily 

 aquatic animals except in fishes. Sense impressions at great distances, 

 as in terrestrial forms, are not possible for aquatic animals. The males 

 of many Lepidoptera are attracted by the odor of the females from 

 distances of several kilometers. Some steppe mammals catch the scent 

 of freshly fallen rain from many kilometers; vultures see and perhaps 

 smell their carrion from an extraordinarily great distance; and the 

 cries of parrots lead monkeys from afar to isolated trees with ripe 

 fruit. 



The much lower conductivity of the air for heat, as compared with 

 that of water, has made possible the appearance of homoiothermal 

 animals among terrestrial forms. It is true that some homoiothermal 

 animals are found in the sea — penguins, whales, and seals, for ex- 

 ample — but they are secondarily aquatic forms with especially well- 

 developed insulation. 



Warm-blooded animals are rendered independent of external tem- 

 peratures, within certain limits; this makes it possible for them to 

 live in zones and at seasons when other animals are dormant with 

 cold. The uniform optimal internal temperature produces, in addition, 

 acceleration of the nerve impulses, shortening of the latent period in 

 muscle contraction, and acceleration of digestion; all of which result 

 in an intensification of the life processes. 



All these factors work together to raise the terrestrial animals to 

 a level of more intense activity and more diversified behavior than 

 that of aquatic animals. This higher level also finds expression in the 

 frequent development of a more complex central nervous system and 

 in the attendant phenomena of instinct and reason. Among primarily 



