39S LAND ANIMALS 



The order of petrels (Tubinares) is associated in its distribution 

 with regions of almost continuous air movement. The powerful alba- 

 tross and the fulmar make use even of the horizontally blowing winds 

 that are deflected from the rising and falling waves. Such birds are 

 absent in calm regions. When the winds are low they may be seen 

 resting on the smooth sea. 



Environmental periodicities. — The climate is rarely uniform in 

 any given place. Of land habitats, aside from caves, it is most nearly 

 so in the dark interior of the tropical rain-forests, where every periodic 

 change in weather conditions is entirely or almost entirely absent and 

 even diurnal changes are minimized. The succession of day and night 

 produces a periodic change in light and darkness, in warming and 

 cooling, and in relative humidity. This has a fundamental effect on 

 animal life. Animals that are directed by their sense of smell, like 

 many insects (for example, the large moths among the Lepidoptera, the 

 lamellicorns among beetles) and many mammals, are independent of 

 the light and can obtain their food by night as well as by day, but ani- 

 mals that are dependent upon their eyes for orientation, like the 

 dragonflies among the insects, and most birds, are forced to rest upon 

 the arrival of darkness. The varying length of the day according to 

 geographical position also influences the conditions necessary for life. 

 Differences in day and night temperatures are often great. In a 

 large part of the tropics, the amplitude of the daily temperature varia- 

 tion is about 6° and is greater than the mean monthly variation which 

 often amounts to only 2°, so that it has been aptly said that the night 

 is the winter of the tropics. In the Sahara, however, and in the Ameri- 

 can desert, the temperature difference between 3 p.m. and 3 a.m. may 

 amount to more than 50°--the nocturnal falling of the temperature 

 makes the desert habitable for homoiothermal animals. The greater 

 humidity of the night, evidenced by the dew, makes it possible for 

 many stenohygric animals adapted to humid air to exist in regions 

 where the daytime conditions are intolerable. Snails that have with- 

 drawn into shells by day come out at night; the Spanish gather snails 

 for food at night by lantern light in places where no snails can be 

 found during the day. Earthworms also are nocturnal. 



Many stenohygric insects, as for example, mosquitoes or stone flies, 

 fly only by night and after rain, since at these times the danger of 

 desiccation is slight. Likewise salamanders, toads, common frogs, and 

 other amphibians wander about principally at night. Thus, the periodic 

 variation of conditions occasioned by the rotation of the earth makes 

 it possible for many animals to frequent certain regions that are un- 

 favorable and hostile for a portion of the day. Finally, the change of 



