CHAPTER X 



STIMULATING EFFECTS OF CROWDING ON 

 THE RATE OF REPRODUCTION 



In a preceding chapter we have seen that there is much support for 

 the conclusion that crowding decreases the rate of reproduction 

 among animals generally, with specific instances among the 

 Protozoa; the Crustacea, of which Daphnia is an example; in the 

 insect Drosophila; and among birds. We have also seen that there is 

 at times an optimum crowding for the growth-rate, which does not 

 necessarily coincide with the minimum population density. It is now 

 necessary to examine whether the evidence that has been advanced 

 demonstrates a similar optimum, at least for certain animals at some 

 time in their Ufe-cycle, in so far as their rate of reproduction is con- 

 cerned. 



The phenomenon which we are to discuss may deservedly be 

 called "Robertson's phenomenon," since Robertson was most active 

 in collecting evidence of its existence. He himself gave it the name of 

 "allelocatalysis," which he defined (1924a) as meaning "the acceler- 

 ation of multiplication by the contiguity of a second organism in a 

 restricted volume of nutrient medium." 



His announcement of the existence of this phenomenon naturally 

 awakened an immediate interest among biologists generally and 

 among students of the Protozoa in particular, many of whom have 

 been unable to confirm its existence. Hence it becomes necessary to 

 examine the development of the problem in an effort to evaluate the 

 results of work centered on it. 



Robertson (1921a, 1923, 1924a, 19246) found that when two in- 

 fusorians, Enchelys and Colpidium (later identified as Colpoda), are 

 introduced into the same restricted amount of fresh culture medium, 

 the early rate of reproduction following a period of readjustment, 

 called the "lag period," is not merely double that shown when a 

 single infusorian of the same species is similarly treated, but is some 



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