CAUSES OF ANIMAL CYCLES 185 



but still lacks absolute proof, and this is perhaps a fair statement also 

 of the relation between JNIicrotus and Toxoplasma in Britain (Findlay 

 and Middleton, 1934; Elton, Davis, and Findlay, 1935). Some of 

 these cases may prove to be similar to that of the chinchbug fungus 

 and grasshopper bacillus. However, the presence of such diseases 

 must be made manifest by all the critical tests of modern bacteriology 

 and pathology before the death of any wild animals can be properly 

 ascribed to them. Rush (1931) says of animal parasites: "Under ordi- 

 nary conditions parasitic infestations of wild animals are not serious; 

 it is only when the animals are concentrated under unfavorable condi- 

 tions that the parasites, figuratively speaking, gain the upper hand." 



The Qualitative or Quantitative Failure of Food Supply. There is 

 perhaps no better example of the destruction of large numbers of 

 animals due to both qualitative and quantitative depletion of the food 

 supply than that of the Kaibab deer (Rasmussen, MS. '32). This was 

 due, however, to an unprecedented destruction of predatory enemies, 

 the cessation of hunting by natives, and very restricted hunting by 

 whites, coupled with the isolated nature of the area which prevented 

 migration. When the cyclic maximum arrived, the already large 

 population led to an enormous increase in numbers. The winter food 

 was depleted, and the herd was reduced from 30,000 to 20,000 in two 

 years (1929-31). Figs. 41-44 show the condition of winter browse 

 and of the evidences of death of deer in 1936. There were probably 

 various causes of the decline in deer numbers, but the lack of winter 

 fodder w-as the principal one. This was accompanied by some dis- 

 ease in the herd. Some fawns were infected with calf diphtheria from 

 which losses in nature were light, and some exhibited the pathological 

 condition popularly known as pink eye (Rasmussen, MS. '32) , but these 

 disturbances were believed to be the result of the malnutrition that 

 caused the decline. This is very different from an epidemic of malig- 

 nant infectious disease. 



Competition for Space and Crowding. Crowding sometimes ap- 

 pears to limit the population, even though food is not involved as a 

 factor. The mere absence of suitable places to go for existence appears 

 to reduce the population. This is shown in experiments by Pearl 

 (1925) and seems evident in quail (Leopold, 1933). Piper brings 

 out the importance of this fact in connection with cycles. 



There remain certain obscure possible causes of decline in numbers 

 and reproductive rate. Allee quoted Kucynski (1928) as follows: 

 'Tn studying the balance of births and deaths among the human 

 population of western Europe, he described the differential effect of 

 changing conditions upon fertility and upon the death rate and con- 



