408 LAND ANIMALS 



than the lack of heat. Many animals can supply their own heat more 

 readily than they can furnish their own water. Homoiothermal animals 

 can endure the winter hardships so long as they find nourishment in 

 sufficient amounts; extreme dryness is injurious to them. In other 

 respects, however, winter and the dry season are very similar in their 

 effects, and are often compared with each other ; both result in marked 

 reduction in plant and in animal activity. As soon as the inhibiting 

 factors are removed, a tremendous awakening occurs. The long- 

 restrained activities of life are again resumed simultaneously in many 

 forms. The majority of animals begin to propagate at once. The spring 

 concerts of male insects, of frogs, and of songbirds, raised against the 

 background of vegetation awakened to new life, form an abrupt con- 

 trast to the desolation just passed. 



The increasing length of the summer day in the higher latitudes 

 permits an increased rate of plant growth which affects all animal 

 life, at least indirectly. We have already seen how profoundly the 

 increase in feeding time affects birds. Similarly, insects such as the 

 bumblebee utilize the entire day in searching for food and in the arctic 

 regions discontinue work for only a short time about midnight. 



Survival through the unfavorable period, be it the dry season or the 

 frost of winter, is the problem which faces the animal population of 

 regions with changing seasons. Certain adaptations become necessary 

 which must be the more complete, the longer the adverse season lasts, 

 and the greater the extremes of aridity or cold. Hence the' number of 

 species that live in such regions decreases, as the living conditions 

 approach the pessimum. 



The great bulk of the invertebrates, the poikilothermal vertebrates, 

 and also many mammals, fall into a sleep-like, inactive condition in 

 protected places during the unfavorable season. Such inactivity is re- 

 ferred to as aestivation if caused by dryness, and hibernation if caused 



by cold. 



Aestivation is common among the snails and may last tolerably 

 long even in mid-latitudes during a summer drought; in snails of hot 

 climates with a continuous dry season, the aestivating period can be 

 prolonged for years at a time without killing the animal. A quiescent 

 period of six years' duration has been observed in Helix veatchii of 

 southern California; 113 H. desertorum from the border of the African 

 desert came to life in the British Museum after five years; ampul- 

 larids, after one to two years. 114 The land planarians and the land 

 leeches of warmer regions spend the dry season buried in the ground. 

 Insects and spiders may aestivate in the steppes; for example, the 

 bumblebees in Corsica and Sardinia, 115 or the solpugid Galeodes in 



