418 POPULATIONS 



better fitted to compete or cooperate with 

 other flocks at the group level than are 

 socially disorganized aggregations. Similar 

 conclusions are suggested by naturalistic 

 evidence. 



There are many strong indications that 

 cooperation at the individual level may 

 also result in groups with increased compe- 

 tence in competition or in cooperation at 

 the group level. Any smoothly working 

 group organization, however achieved, is 

 helpful under many conditions. Probably 

 the relations between individuals forming 

 simple groups are repeated when such 

 groups as social units become compounded 

 into a more complex social order. Even 

 when society becomes still more involved, 

 unit-to-unit cooperation— or competition, if 

 not too severe-may lead to group organi- 

 zation that increases the effectiveness of the 

 larger unit in its competitions and coopera- 

 tions (cf. Collias, 1944; Allee, 1945). 



SUMMARY FOR NATURAL 

 COOPERATION 



Having brought the discussion of group 

 organization to the point at which survival 

 values have been considered, it is now fit- 

 ting that we draw together many of the 

 threads of thought running through this 

 chapter by considering the evidence of nat- 

 ural cooperation in summary form. This 

 summary may also serve as a partial sub- 

 stitute for a mass of data that cannot be 

 presented here in detail. The evidence, 

 however, cHnches, with Darwin-hke thor- 

 oughness, the preexperimental insight of 

 Espinas (1877), Wheeler (1923) and 

 others (History, p. 30) and the conclu- 

 sions of Allee (e.g., 1947) based on ex- 

 perimental as well as naturahstic evidence, 

 that natural, unconscious mutualism is one 

 of the basic principles of biology. 



1. At all levels in the animal kingdom, 

 and under a variety of conditions, there is 

 added safety in numbers up to a given 

 point. There is danger also in overcrowd- 

 ing, but it is the ill effects from under- 

 crowding that give the most generalized 

 evidence for natural cooperation or at least 

 for proto-cooperation among hving organ- 

 isms. Macerated cells of a sponge will not 

 develop if too few are present, and the 

 smallest embryonic transplants often fail to 

 grow when somewhat larger ones succeed. 



If a population in nature becomes reduced 

 to a few individuals, it is in danger of 

 dying out, even though apparently able to 

 persist. 



2. Many plants and animals are able to 

 modify an unfavorable environment to 

 such an extent that, though some or all of 

 the pioneers may be killed, others follow- 

 ing and some associated with them can 

 survive and even thrive when they could 

 not do so in a raw environment. 



3. Certain vital processes are adaptively 

 retarded by increased numbers up to a 

 given population density. For example, 

 scattered spermatozoa of many marine 

 animals lose fertifizing power more rapidly 

 than they do when massed together. 



4. Other biological processes are benefi- 

 cially accelerated in the presence of popu- 

 lations of optimal size and density. Such 

 processes are retarded both with oversparse 

 and with overcrowded populations. The 

 cleavage rates in sea urchin eggs and cer- 

 tain other aquatic eggs follow this rule. 



5. Various kinds of Protozoa show an 

 acceleration in rate of asexual reproduction 

 with a medium rather than a sparse popu- 

 lation density. Similar phenomena may 

 have been a forerunner of the evolution of 

 sex that, according to this attractive hypoth- 

 esis, grew out of certain proto-coopera- 

 tions of asexual organisms. Once evolved, 

 sexual relations have played a large part 

 in the further development of the social 

 life of animals, including man. 



6. Colonial Protozoa could hardly have 

 arisen from solitary forms unless the colony 

 of cells that remained attached to each 

 other after divisions had shown survival 

 values over and above those exhibited 

 when the cells were scattered singly. 



7. The evolution of the many-celled ani- 

 mals, the Metazoa, from the Protozoa was 

 probably based on similar relationships. 



8. Each advance in complexity of meta- 

 zoan individuals came from the natural 

 selection of an increased ability in natural 

 cooperation on the part of the evolving 

 stock. 



9. Charles Darwin recognized that a rel- 

 atively large population is a highly impor- 

 tant factor in evolution by natural selection. 

 There is more recent evidence that evolu- 

 tion proceeds more rapidly and certainly 

 in populations of interbreeding animals 

 that are not too small. 



