CHAPTER VII 



RETARDING INFLUENCE OF CROWDING ON 

 THE RATE OF REPRODUCTION 



In the preceding chapter we have assembled evidence to demon- 

 strate that among many animals overcrowding tends to produce 

 dwarfed individuals, and have discussed the factors that have been 

 suggested as operating to produce this effect. As might be expected, 

 there is frequently a retardation of the rate of reproduction as well 

 as of the growth-rate of the individual. In many respects the two 

 phenomena overlap. The evidence for the slowing-down of repro- 

 ductive rate under crowded conditions will be examined in part in 

 the present chapter. At another place consideration will be given to 

 the data brought forth by Robertson and others which indicate that 

 under certain conditions the rate of reproduction is increased in early 

 stages of protozoan or other cultures when more than one animal 

 is present in a limited amount of medium. 



REDUCED DIVISION RATE IN INFUSORIA 



Balbiani (i860) reported from a single experiment on Parame- 

 cium that this infusorian must be in not less than 2-3 cc. of medium 

 for the greatest productivity to be realized. Kulagin (1899) sug- 

 gested that this is due to the accumulation within the medium of 

 excretions analogous to toxins, which gradually accumulate until the 

 nucleus is affected. 



Woodruff took up this problem in 1911 in an effort to find the 

 effect of excretion products of Paramecium on its rate of reproduc- 

 tion. Since the experiments of Woodruff usually form the starting- 

 point for present-day citations on this subject, they deserve to be 

 given in some detail. 



The reproduction of P. aurelia was followed for from 16 to 20 days 

 in four volumes of hay infusion: 2, 5, 20, and 40 drops, which were 

 changed at 24- and 48-hour intervals in different series of experi- 



