No. 643] 



EFFECTS OF ENVIRONMENT 



155 



marily upon the motion of the earth and moon. As the 

 earth makes its annual journey around the sun, the ani- 

 mals of temperate and polar regions, and to a less extent 

 those in the tropics, are subjected to seasonal changes in 

 environment. These changes are related chiefly to tem- 

 perature, available moisture, and food. Animals gen- 

 erally respond to such environmental variations by 

 adjusting appropriate activities to favorable times. In 

 general winter is a season for resting ; spring, for mating 

 and propagation; summer, for feeding and growth; and 

 autumn for fructification. Seasonal succession is a succes- 

 sion of stages in life cycles. The seasonal rhythm has a 

 short enough period to permit animals to become adapted 

 to it. Their systems of activities vary to fit the seasons. 

 Every one is familiar with the seasonal migrations ot 

 animals. The arctic tern travels from pole to pole, and 

 thus always lives in sunshine. ]\lany animals do not 

 migrate, but pass the winter in a dormant condition. In 

 the tropics animals frequently tTstivate during the annual 

 dry season. Now many of these seasonal responses arc 

 certainly due to stimuli received from the environment. 

 The littie Daiihnias, that live in fresh-water habitats the 

 world over, usually liave long helmets in summer and 

 short helmets in winter, but long-helmeted forms can ])e 

 made to i)roduce sliort-hehneted offspring in summer by 

 keei)ing them at low tempers tures. In this instance the 

 effective stimuhis n])].oni-s to ))e tlieriiial in nnturo. But 



respondinu' ;i> f*;!i- .'i- they abh' to stininli that come 

 with rlixthmi.- .•luiiiur^ ^^lth..ut tlicir bodii-. Tli.' living 

 system apparently has a rliytlini of its own that is adapted 

 to the seasons. Smallwood'" kept a fciualc dou'lish [Ann.i 

 calva LinnaBUs) in an aquarium, practically witluuit Whh]. 

 for twenty months at rather constant t('iiii»ci'atin'c. Dnr- 



