408 



POPULATIONS 



of evolution becomes a criterion for the ex- 

 istence of natural cooperation. 



GROUP MODIFICATION OF STRUCTURE 

 AND BEHAVIOR 



In addition to the many instances in 

 which animal aggregations have demon- 

 strable survival value, only a few of which 

 have been reviewed in the last several 

 pages, aggregations of plants or animals, or 

 of both, often aflFect the behavior, the phys- 

 iology, and even the structure of the con- 

 stituent organisms. Special survival values 

 associated with such modifications are not 

 always readily apparent and may be non- 

 existent. A few examples will indicate the 

 range of this general category of group 

 efiFects. 



As with trees and other plants, the 

 growth form of many sessile animals de- 

 pends on the degree of crowding; barnacles 

 and ascidians furnish diagrammatic ex- 

 amples. The size and details of structure 

 of Drosophila are affected by population 

 density (Eigenbrodt, 1925; Plunkett, 

 1926). The polarity of the zygote of the 

 alga Fticus may be determined wholly or 

 in part by the relation of a given egg to 

 its fellows (Rosenvinge, 1889; Hurd, 1920; 

 Whitaker, 1931). With certain aphids wing 

 production is influenced by the degree of 

 crowding (p. 347). 



The whole modern phase theory of lo- 

 custs maintains that certain species of or- 

 thopterans are polymorphic and that struc- 

 tural proportions, coloration, and behavior 

 are determined by population density 

 (Uvarov, 1928; Faure, 1932). 



Space does not permit an adequate re- 

 view of the many instances in which the 

 activities of animals are modified by the 

 presence or absence of other animals. It 

 may be sufficient to mention the behavior 

 related to courtship and mating. We do 

 not know of an adequate summary of this 

 important aspect of ecology. Such a treat- 

 ment could begin with the mating behavior 

 and the mating types of Protozoa (Jen- 

 nings, 1945; Sonnebom, 1947) and, if 

 thoroughgoing, would draw illustrations 

 from almost all animal phyla. The article 

 on "Courtship of Animals" by Julian Hux- 

 ley in current printings of the Encyclo- 

 paedia Britannica gives an elementary in- 

 troduction to this field. 



ANIMAL AGGREGATIONS AND SEX 



The whole gamut of the sex-related hfe 

 of adults makes significant contributions to 

 several aspects of animal ecology and gen- 

 eral sociology. Sex-based behavior is seen 

 to be the more important when the rich 

 variety of parent-offspring relations are con- 

 sidered. Sex itself may have originated in 

 the protista as a result of selection acting 

 on the mutual stimulation to more rapid 

 division that often accompanies the close 

 contiguity of two asexual individuals. This 

 hypothesis is most appropriate for those 

 cases of so-called allelocatalysis (p. 357), 

 in which the accelerated division rates ap- 

 pear to stem from the production and auto- 

 matic sharing of a growth-promoting sub- 

 stance (Robertson, 1927; Reich, 1938; 

 Kidder, 1941; Mast and Pace, 1938, 1946). 



Even when bisexuality is fairly estab- 

 lished, sex determination is often flexible 

 and, in a number of instances widely sep- 

 arated in the animal kingdom, depends on 

 the closeness of association with one or 

 more other organisms of the same species. 



Conditions described for the alga-like 

 flagellate, Chlamtfdomonas eu^ametos, by 

 Moewus (e.g., 1933, 1940) and associates 

 seem to lend support to such a suggestion, 

 which was independently worked out from 

 Robertson's effect (allelocatalysis) in cili- 

 ates (Allee, 1931). Certain phases of the 

 studies on Chlamtfdomonas have been sub- 

 jected to strongly adverse criticism on 

 statistical grounds (Philip and Haldane, 

 1939). The more strictly ecological aspects 

 of the situation have been reviewed with 

 care by Sonnebom (1942) and by G. M. 

 Smith (1946). The last-mentioned worker, 

 using Chlamtfdomonas from California, 

 failed to confirm some of the experimental 

 results reported for another species from 

 Germany. The further development of this 

 particular problem should be watched. 



Mating types are known for certain 

 Protozoa and have been much studied in 

 some of the common species of Parame- 

 cium and in Etiplotes (Sonneborn. 1937, 

 1947: Jennings, 1938. 1945; Kimball, 1939, 

 1943). At first the situation appeared to 

 have a simple regularity and was ouite 

 dramatic. Each of several species was found 

 to be divided into a number of mating va- 

 rieties, each containing a limited number 

 of mating types; there were two for 



