122 ANIMAL AGGREGATIONS 



the substances which Paramecia excrete into their medium are es- 

 sentially species-specific, at least to the extent that they do not uni- 

 formly influence the rate of reproduction of the hypotrich Stylony- 

 chia. This hypotrich produces conditions within its own culture me- 

 dium which are definitely depressing for hypotrich reproduction and 

 without necessarily affecting the rate of reproduction of Paramecium. 



The question of species specificity has not attracted the work it 

 deserves; but, stimulated by the researches of Robertson, to be re- 

 ported in a later chapter, several workers have retested the effect of 

 crowding upon the rate of reproduction of Paramecia and of other 

 protozoans. Without exception, all the workers reporting so far, 

 Robertson included, have confirmed the conclusions reached by 

 Woodruff for cultures running the length of time for which his 

 averages were taken. A detailed discussion of this later work is post- 

 poned for the present. 



From general considerations it appears highly probable that the 

 relationships outlined above and in the preceding chapter, if properly 

 adjusted, could so affect an animal (for example, Paramecium) that, 

 while it might be able to continue to live, its powers of reproduction 

 would be lost. Crampton (191 2) tried this experiment. He found 

 that a single Paramecium confined in a capillary tube could be kept 

 from fission for as long as 32 days, while controls relatively un- 

 restricted as to space were dividing at a rate that would produce 

 4,300,000,000 animals in the same time. He recognized three factors 

 as working to produce this effect: lack of sufficient nutrition, ac- 

 cumulation of waste products, and stimulation from the narrow 

 limits. That lack of sufficient or proper food is not the sole cause is 

 shown by his experience that the confined animals could be released 

 to swim about in a culture of Bacterium iermo daily for as long as 1 2 

 hours out of the 24, without division, if the remainder of the day 

 were spent in the confinement of the tube; and that they could be 

 held so without division for a week, while controls were dividing on 

 an average of once a day. Such Paramecia remained plump and well 

 nourished in appearance ; those left in the tubes for long periods with- 

 out changing became transparent and emaciated. Stylonychia gave 

 similar results. It is significant that Crampton centrifuged his ani- 



