BIOTIC BALANCE 173 



out the fact that such fluctuations occur in the sea relative to animals 

 little influenced by man's activities (Blegvad, 1925) and in the arctic 

 regions where such influences have affected only some of the larger 

 species. This knowledge has been responsible for a new view of 

 biotic equilibrium which assumes, as normal, a rather wide fluctuation 

 in the number of individuals for many species. Frequently, several 

 species rise to a maximum or sink to a minimum at about the same 

 time. The abundant species react upon the habitat and destroy or 

 provide food for other organisms, but the onslaught on the excess 

 population due to flexibility of food habits, or the pressure of climate 

 and the limitations of shelter and food supply, are the important fac- 

 tors in checking the increase of various species. In addition, disease 

 may become a factor. 



Elton (1930:17) is a strong advocate of a continuing state of lack 

 of balance in nature, as the following excerpts indicate: 



" The balance of nature' does not exist, and perhaps never has 

 existed. The numbers of wild animals are constantly varying to a 

 greater or less extent, and the variations are usually irregular in 

 period and always irregular in amplitude. Each variation in the 

 numbers of one species causes direct and indirect repercussions on the 

 numbers of the others, and since many of the latter are themselves 

 independently varying in numbers, the resultant confusion is 

 remarkable. 



'T shall take from this region one example, which illustrates very 

 clearly the vast degree of unbalance that nature presents to the un- 

 aided eye. . . . Any animal community that is watched for a number 

 of years presents a restless picture of unending changes in numbers, 

 which are more often than not accompanied by striking changes in 

 their habits of life. 



"This instability is partly due to the external influences, and partly 

 to the inherent manner in which every animal community is con- 

 structed. The environment (and in particular the climatic factors 

 of the environment) does not remain the same from day to day and 

 year to year. These changes react on the animals either through 

 their direct effect or indirectly by affecting the plant-life of the region. 

 On the other hand, internal factors dependent upon the manner in 

 which animal communities are organized, also play a part in upset- 

 ting the balance of numbers. Epidemics caused by parasites are a 

 very important example of this, while the change of habits consequent 

 on the scarcity of food forms another. Irregular migration is one 

 of the most important of all." 



Criddle (1930) has pointed out a series of interactions connected 



