190 AGGREGATION, COMPETITION AND CYCLES 



sary a comparative study of stations distributed over a sufficiently 

 large area to eliminate the possibility of erroneous conclusions being 

 drawn from quantitative observations made at unfortunate locations. 

 An example of short-period effects is presented by Seamans (1926), 

 who found that the number of rainy days in a very brief period prac- 

 tically controlled the abundance of the pale western cutworm (Sas- 

 katchewan and Alberta) . Thus, if there are less than 10 wet days in 

 May and June, the number of cutworms will probably increase the 

 following year; if there are between 10 and 15 wet days, there will be 

 a decrease. 



The Rise to Maximum Abundance. The unusual reproductive 

 capacity of the chinchbug at certain times has already been noted 

 (page 189). It is evident that successful reproduction is sometimes 

 greatly accelerated above the maximal expectancy, as Collett has indi- 

 cated for the lemming (1895). Before the exceptional emigration in 

 the autumn of 1891, the number of litters increased to 4 during the 

 summer, the first appearing very early in the spring. The greatest 

 number per litter was found in the second, with a frequent maximum 

 of 11; the third litter averaged 5 or 6 with 10 as the extreme, while 

 the rule was 5-7 for the fourth. In the case of pregnant young 

 females the number of the first litter was reduced, ranging from 3 to 6 

 (cf. Heape, 1931:81). Manniche (1910) holds the opinion that skuas 

 and snowy owls breed chiefly or solely during the lemming maximum, 

 while an even more striking illustration of the cycle sequence is af- 

 forded by the short-eared owl and peregrine falcon in south Norway. 

 The former gathers in large numbers and feeds upon the hordes of 

 lemmings, while it is then preyed upon by the falcon, which migrates 

 for that purpose (cf . Elton, 1924) . 



Apart from Heape's view as to the role of vitamins (1931:95), 

 there has been little consideration of the part specifically taken by 

 metabolism and by hormones. The significance of these in the migra- 

 tion of birds creates a strong presumption that they are likewise of 

 importance in the reproductive and migration cycles of other animals, 

 primarily in connection with the maximum. 



Migration is not an uncommon feature at the time of the maximum 

 or near it, and it is not improbable that it bears something of a 

 causal relation to the minimum in such cases. In striking instances 

 such as those of the European lemming, this is true emigration 

 (Heape's sense), perhaps due to overcrowding and the need for food, 

 but with some, if not all, of the birds, whether herbivores or carnivores, 

 the process is remigration at more or less cyclic intervals. As to the 



