72 THE ECOLOGY OF ANIMALS 



on the introduced American grey squirrel in England 

 (1932) and of Harrisson and Hollom (1932) on the 

 great crested grebe in England, have shown that 

 spreading takes place equally in years of abundance 

 and years of scarcity ; in other words, that a great 

 deal of spreading may be due to accumulated local 

 movements at the edge of the range, and not to 

 pressure of numbers in the ordinary sense. 



In the study of population problems two other 

 aspects that have not so far been mentioned are of 

 importance. The first is reproduction, and the 

 second the length of life and age distribution of the 

 population. Reproduction and the factors influenc- 

 ing it are mainly physiological problems, but they 

 set the conditions for multiplication of any species. 

 Ecologists have carried out important work on the 

 influence of climatic and other factors on reproduc- 

 tion (see Chapman, 1931, and Uvarov, 1931), while 

 the effects of undercrowding and overcrowding on 

 animals have been summarized by Alice (1931). For 

 mammals and birds there is a growing literature on 

 the factors controlling breeding seasons, and the 

 recent experiments of Rowan (1929) on birds and 

 Baker (1932) on voles have shown the importance 

 of the length of daylight and other less unexpected 

 factors in controlling breeding. Baker (1929) has 

 also studied the curious periodicity of breeding 

 seasons in equatorial animals living under apparently 

 constant conditions. 



The second important variable — length of life — is 

 of importance in fixing the replacement rate neces- 

 sary to maintain or increase any population. It 

 partly determines the rate of turnover of food stuffs 

 and the living matter formed from them. Little 

 accurate information exists about the length of life 

 of wild species. Pearl and Doering (1933) have 

 pointed out that such information is best charted in 

 the form of a ' life curve ' of the kind used by 

 insurance companies. This life curve shows the 



