UNBALANCE IN NATURE 95 



fluctuations, we are in an entirely different world. 

 The situation resembles the shift from the uniform- 

 ity of evidence that there are cycles in human 

 business to the dissension concerning the causes 

 of these cycles. The causes of cycles in numbers 

 of animals appear to be divided into three groups : 

 biological, climatic and astronomical. Among 

 the biological causes we must suspect variations 

 in food, in reproductive rate, in parasites, both 

 bacteria and animal, but particularly in the food. 

 Regarding variation in food production, I have 

 already indicated that this may play an important 

 role. There is some evidence that the rate of 

 reproduction varies in a somewhat cyclic manner, 

 beyond the annual cycle; thus the snowshoe rabbit 

 of Canada tends to produce larger litters when 

 food is plentiful and smaller ones when conditions 

 are severe. The importance of parasites has 

 probably been under-emphasized. The layman 

 particularly is happily unaware of the enormous 

 numbers of animal parasites which infest most 

 forms of wild life, plants included. Thus there 

 is evidence that the growth of the forests of 

 Germany is influenced as much by their insect 

 pests as by temperature and rainfall. An even 

 greater role in determining population numbers 

 among animals is played by bacterial parasites. 

 Apparently these undergo fluctuations in viru- 



