410 



POPULATIONS 



genetic females. Crowding females of 

 Moina is an effective method of inducing 

 bisexuality. Crowding acts both by decreas- 

 ing the amount of available food (Stuart 

 and Cooper, 1932) and, less certainly, by 

 the concentration of wastes or of their de- 

 composition products (Banta and Brown, 

 1939), or, perhaps, by both these mecha- 

 nisms combined with other factors as yet 

 unanalyzed. Whatever the explanation, in 

 Moina macrocopa the close crowding of 

 many parthenogenetic females results in 

 the production of eggs with a different 

 prospective potency from that found in 

 other eggs produced by these same females 

 when uncrowded. 



The survival values of adaptations that 

 tend toward the production of males, sex- 

 ual females, and the resulting resistant 

 eggs, under adverse conditions, are rather 

 obvious. All individuals of the uncrowded 

 parthenogenetic stock are offspring-produc- 

 ing members of the population when en- 

 vironmental conditions favor rapid expan- 

 sion. By using the type of reasoning com- 

 monly employed in dealing with problems 

 of adaptive evolution (p. 630), each of 

 the other instances cited can also be shown 

 to have certain probable survival values. 

 Thus, as a result of the speciahzed method 

 of sex determination described for the 

 worm Bonellia, the potential reproductive 

 waste of isolated males is largely avoided. 

 If a young, wandering, sexless individual 

 reaches a suitable environment, it develops 

 into a female and is then able to direct the 

 transformation of the next comer into a 

 functional male. The survival values of the 

 sexual situation in Crepidula are somewhat 

 similar; those connected with the popula- 

 tion control of sex ratios of nematode para- 

 sites are more compHcated and are related 

 perhaps, to the need for avoiding over- 

 parasitization of the hosts if the parasites 

 are to flourish. 



SOCIAL FACILITATION 



The nearby presence of another organism 

 frequently modifies the rate of performance 

 or even the character of a physiological 

 process or a behavior pattern. Certain of 

 these interrelations were recognized by 

 Tarde (1903) as phases of interphysiology 

 or interpsychology, ideas that can be read- 

 ily expanded to include intermores phys- 

 iology or intermores psychology. All such 



concepts enter into social facihtation. The 

 distinction between physiology and psy- 

 chology is made on the simplified, but per- 

 haps truthful, assumption that psychology 

 is an aspect of the physiology of the cen- 

 tral nervous system, especially of the higher 

 centers of the cerebral ganglion or brain. 



In general, social facilitation refers to 

 any increment or decrement in an individ- 

 ual's behavior resulting from the presence 

 of another organism. It is one of the funda- 

 mentals of group physiology. Social facih- 

 tation usually implies an increase in fre- 

 quency, intensity, or skill; it may also refer 

 to an increased tendency to remain quiet. 

 Proto-cooperation may be helpfully re- 

 garded as incidental or fundamental phys- 

 iological facilitation. Schneirla (1946) 

 treats this in close connection with the "bio- 

 social facihtation" found in social insects 

 and the "psycho-social facihtation" illus- 

 trated by man; he thinks that both may be 

 considered as extensions of trophallaxis as 

 outhned by Wheeler (1923). These three 

 phases are not sharply separated; all are 

 shown by the high primates, for example, 

 and probably represent aspects of social 

 evolution (cf. p. 687). 



The more extreme results of social facih- 

 tation range from antagonisms to syner- 

 gisms; and in the twifight intermediate 

 zone, reactions may be subtly expressed by 

 variation in rate of response. Synergisms 

 are well known among bacteria (Burrows, 

 1942), among other plants, and through- 

 out the animal kingdom. Social facihta- 

 tions are especially striking among many 

 insects and vertebrates. The effect may be 

 produced as a response to an altered phys- 

 ical environment or as a direct reaction to 

 the presence of other organisms. 



Let us restrict our attention to the last 

 two categories, and to behavior. Many 

 diverse animals show retarded rates of 

 learning when another similar form is 

 nearby. Thus the common cockroach learns 

 to run a simple maze more expertly if alone 

 than if another is present (Gates and Alice, 

 1933); the shell parrakeet behaves similar- 

 ly (Allee and Masure, 1936) and does not 

 learn to talk human words if other budge- 

 rigars are present (Feyerabend, 1943). 



The rate of respiration and the amount 

 of movement may be automatically reduced 

 in certain fishes when another similar fish 

 is nearby (Shlaifer, 1939). Attention has 



