i 9 4 THE VARIATION OF ANIMALS IN NATURE 



buoliand) that the insect parasites account for about 60 per cent, 

 of the larvae. In all insects death from unfavourable climatic 

 conditions is also very frequent in the early stages, so far as the 

 facts have been recorded (Uvarov, 1931). Kirkpatrick (1923) 

 has provided an elaborate account of the Egyptian cotton- 

 seed bug (Oxycarenus hyalinipennis) . At the end of the breeding 

 season this insect may be present at the rate of 7-12 millions 

 per acre, while at the end of the winter not more than 100,000 

 per acre are left. During the whole of his work no parasitic 

 or predacious enemies were discovered, and all effective control 

 appears to result from the operation of normal weather con- 

 ditions. Sunlight kills some of the eggs, and some of the young 

 nymphs die, possibly through lack of moisture or failure to 

 penetrate the boll quickly enough. Heavy rainfalls and the 

 harvesting of the bolls account for many more. During the 

 winter the death-rate from drought must be enormous, especi- 

 ally as many of the bugs leave their hibernacula on warm days 

 and probably fail to regain suitable quarters when the weather 

 alters. Yet, in spite of its rather imperfect adaptation, this 

 species can maintain itself in great abundance. 



Russell (1932) has summarised some of the data as to the 

 fluctuations of certain marine organisms. The populations of 

 bottom-living Mollusca seem to undergo extreme variation, 

 and in certain cases it is thought that this is due to variations 

 in the course of currents by which the larvae are carried 

 passively. When the larvae settle down, only those survive 

 which happen to have drifted over areas of suitable bottom. 

 The very large mortality amongst those which have been 

 carried to unsuitable areas must be largely random. It would, 

 in fact, appear to be a general rule that the more directly 

 dependent an organism is on its environment, the larger will 

 be the element of chance in the death-rate. 



In many mammals, as Elton's well-known studies have 

 shown, the decimation of the population is a periodic pheno- 

 menon. A period during which the death-rate is relatively 

 low culminates in an enormous increase in numbers, leading 

 in turn to a catastrophic reduction, often as a result of an 

 epidemic. Many examples are given by Elton in his book, 

 1 Animal Ecology and Evolution ' (1930, pp. 19-23). 



It has been argued (e.g. Muir, 1931) that because 90 per cent, 

 of the individuals perish before reaching maturity, a selective 



