10 



NATURAL HISTORY 



(,?) ir. L. Macchtlin 

 During the freezing weather in January, 1937, in California, citrus groves 

 which were adequately protected by heaters lost relatively little fruit, while many 

 unprotected groves suffered a complete loss of fruit as well as some trees. 



much more rapidly at warm than at cold temperatures, and this is true 

 of the reproduction of many animals. Many tropical animals may 

 withstand cold temperatures, but will not propagate at those tem- 

 peratures. H. B. Ward ^ has made observations on the sockeye 

 salmon which indicate that these fish in swimming up rivers to spawn 

 always take the river of slightly cooler temperature, a difference of 

 1° F. being sufficient to divert the fish. Seasonal cycles of activity 

 are largely influenced by temperature, this being particularly true of 

 reproductive activity, which plays a part in the migrations of birds, 

 the rapid multiplication of plankton and other forms. Some animals 

 respond to a cold temperature by going into a resting state or hiberna- 

 tion, while others go into a dormant condition because of unfavorable 

 conditions of heat and dryness. This latter state, aestivation, is often 

 seen in regions having marked periods of alternating rain and drought. 



I Ward, H. B. " Some Responses of Sockeye Salmon to Environmental Influences during Fresh- 

 water Migration." Ann. and Mag. of Nat. Hist., Vol. VI, pp. 18-36. 



