144 ANIMAL LIFE AND SOCIAL GROWTH 



turbance of expected ratios of different sorts of 

 offspring. If the larvae of such carefully selected, 

 pure-bred flies are over-crowded, differences ap- 

 pear not only in weight and length of the whole 

 animal, which would be expected, but also in the 

 proportions of body parts. The number of facets 

 in the eyes, the number of hairs, the number of 

 teeth in the "sex-comb" of the male, are all 

 affected by crowding. There is some evidence 

 that too small a population of flies produces some 

 changes similar to those produced by population 

 numbers above the optimum. In general it has 

 been found that unless at about their optimum 

 population, it is unsafe to draw quantitative 

 conclusions concerning the effect either of heredi- 

 tary or of other environmental factors. 



From this evidence it is apparent that profound 

 as well as superficial changes in bodily structures 

 of animals as well as of plants may be produced 

 by crowding. Taken together with the findings 

 reported in the preceding chapter, we are prepared 

 to support the contention of other research 

 workers that: "In general there can be no question 

 that this whole matter of influence of density of 

 population in all senses, upon biological phenom- 

 ena, deserves a great deal more attention than 

 it has had. The indications all are that it is the 

 most important and significance element in the 

 biological as distinguished from the physical, en- 

 vironment of organisms." 



