ii6 ANIMAL ECOLOGY 



carrying out a survey of the islands on Lake Victoria in order 

 to discover the distribution and ecology of the tsetse fly, noted 

 that islands below a certain size did not support any flies at 

 all, although the conditions for breeding and feeding (v^hich 

 are well defined and regular) were otherwise apparently 

 quite suitable.^* The explanation of this was probably that 

 the fly population is subject to certain irregular checks upon 

 numbers, and that any one population must be sufficiently 

 large to survive these checks. There would not be a big 

 enough margin of numbers on a very small island. 



There are suggestions of a similar state of affairs among 

 certain protozoa. It has been found that if there are too few 

 individuals in a culture they do not live so successfully,^! and 

 this is also said to be true of cells growing in tissue- cultures. 

 Again, it has been found that the minimum density is not the 

 optimum density for a population of the fruit-fly Drosophila 

 growing in the laboratory. ^^ It is quite probable that there 

 are sometimes physiological or even psychological reasons 

 controUing the desirable density of population, just as it is 

 bad for most people to Hve alone, or, on the other hand, under 

 too crowded conditions. But we do not know much about 

 this matter among wild animals. It may be of very great 

 importance in their lives and cannot be ignored as a possible 

 factor affecting numbers. 



24. Before going on, it will be convenient to sum up what 

 has been said so far about the numbers of animals. Most 

 animals are more numerous than is usually supposed, and it 

 is necessary to accustom the mind to deaHng with large actual 

 numbers of individuals. One is the more likely to under- 

 estimate the numbers of animals, owing to the destruction of 

 the large and more conspicuous species which were formerly 

 so much more abundant in many parts of the world, now 

 occupied by industrial or agricultural civilisation. Descrip- 

 tions of the enormous numbers in which these larger animals 

 still exist in the more secluded parts of the world and 

 of the former numbers of animals which are now rare 

 or extinct, enable us to grasp to some extent the vast 

 abundance of the smaller and more inconspicuous forms. 



