782 CONCLUSION xxxn. 15- 



and all the time. The number of animals in a population is seldom if 

 ever constant. Probably the genetic and phenotypic characters of the 

 members also vary with time, perhaps actually in correlation with the 

 fluctuations of number. 



We have, therefore, a picture of an animal species as a set of indi- 

 viduals that are similar but not all identical, interbreeding with each 

 other, though perhaps with degrees of difficulty that vary with genetical 

 and geographical differences (themselves probably correlated). The 

 characteristic features of such a population are given, as we have seen 

 in Chapter I, by its power and ability to produce later populations, 

 both like and unlike itself. Unfortunately, we know little about these 

 powers in vertebrates, or indeed other organisms; the present study 

 has not dealt with them fully. They are presumably influenced by such 

 variables as the frequency of reproduction, number of offspring pro- 

 duced, and the viability of these in the face of various climatic and 

 biotic factors, availability of food, persistence of predators, and so on. 

 It has been pointed out by Haldane that these factors may lead to the 

 development of various distinct sorts of organization. If the death- 

 rate is low the productivity will also be low and those members of the 

 population will be selected whose characters allow for a long life — for 

 example, hypsodont molars will develop. In populations with a high 

 death-rate to predators or disease, however, selection will choose those 

 individuals with high productivity and rapid development, incidentally 

 perhaps also allowing greater variability, by which the predators and 

 pathogens may be avoided. 



There must be a complicated relationship between such factors as 

 the frequency of reproduction and numbers of young, rate of growth, 

 time of maturity, size, likelihood of death from predators and patho- 

 gens, capacity to 'adapt' during the lifetime and especially to store 

 information in the nervous system. Members of species that breed fast 

 usually also grow fast, learn little, and are killed before they grow old. 

 Where there is a brain with a large memory it usually directs a massive 

 individual and keeps it alive for a long time. 



Evidently the particular characteristics of a population (including 

 its productivity) will depend on the influences to which it is subjected. 

 Many of these variables act with an intensity that depends on the 

 activity of the adult organisms and the energy and ingenuity with 

 which these find situations suitable for the life of themselves and their 

 offspring. Since this adult activity is itself influenced by hereditary and 

 by environmental factors it is clear that the productivity and increase 

 of a population depend on a very complicated system of inter-connected 





