376 



GENERAL BIOLOGY 



of environment, and that result in a succession of floras, 

 may be studied if there be available a series of 

 ponds in the various stages of filling, or if 

 there be burned over tracts or fallow fields in 

 the various stages of reforestation. Sugges- 

 tions for such studies may be found in a num- 

 ber of modern text books of botany. The 

 adjustment for geologic time is studied in the 

 palaeontologic record, and is the history of 

 plant life on the earth from the beginning. 



2. Hibernation and aestivation. 



Corresponding to the seasonal adjustments 

 of early and late plants, just cited, there is 

 seasonal cessation of vital activity among 

 animals. In our temperate climate, many 

 warm blooded mammals, and most other 

 resident animals, disappear on the advent of 

 cold weather, and may be found in a dormant 

 condition, in winter quarters. They are hiber- 

 nating. Their temperature is barely above 

 freezing point. Their metabolism is well 

 nigh at a stand still. In the spring they emerge 

 in good condition and resume their wonted 

 activities. Nature effects great economy by 

 limiting their foraging operations to the grow- 

 ing season. 



On the other hand, in the hot weather of 

 summer, with its accompanying drouth, when 

 there is not enough water to maintain activity 

 on the part of organisms that live in temporary 

 shoals there results another resting stage that 

 is knovvn as aestivation. Thus, through the 

 central United States the damselfly shown in 



FIG. 219. The 

 aesti v a t i n g 

 embryo of 

 L e s t e s , as 

 seen through 

 the translu- 

 cent egg 

 shell. I, la- 

 fa r u m; m, 

 antenna; n, 

 mandible; o, 

 maxilla; p, 

 labium; q, r, 

 s, legs of one 

 side; t, abdo- 

 men. 



