NUMBERS : DYNAMICS 73 



average number of individuals which survive to each 

 age, and therefore also expresses the probable age 

 distribution of the population at any moment. This 

 age distribution, as has been mentioned, is of impor- 

 tance in controlling to some degree the density of 

 parasites in a mammal population. Such life curves 

 have been determined for one species of ' wild ' 

 animal in nature (man) and for at least two species 

 of animals under laboratory conditions (the fly 

 Drosophila and a rotifer Proales). 



Our present knowledge of the dynamics of animal 

 populations may thus be summed up as follows. 

 The highest density for a species is not one which 

 can long be maintained in nature, owing to the 

 operation of automatic biological factors which bring 

 about reduction in numbers, often far below that 

 which could be permanently maintained. To a 

 certain extent it is also true that very low densities 

 cause a slowing down of increase in numbers. Reduc- 

 tion in numbers is also brought about very frequently 

 by extraneous factors such as climatic variations, 

 which cause catastrophes whose extent does not 

 depend in any way upon the density of numbers at 

 any particular time. Mathematical- biological theories 

 have been developed to show that even in a constant 

 physical environment fluctuations in numbers are to 

 be expected in animal communities, these being 

 caused by the inter-relations that exist between 

 animals. If a single species of herbivorous animal 

 free from enemies or parasites were to be kept in a 

 constant environment, we should expect its popula- 

 tion to increase either up to the exhaustion of food 

 supply, or to supply its own intra-specific checks. 

 The latter have been shown to exist in the flour 

 weevil {Tribolium confusum) studied by Chapman 

 (1928) and Holdaway (1932). Here the final level 

 density is brought about by the fact that the weevils 

 devour their own eggs if they chance to meet with 

 them. A certain density of eggs and of weevils 



