360 



POPULATIONS 



is removed its full effect is noted" (p. 

 225). 



Finally, should the reader wish to con- 

 sult a treatment of the "allelocatalytic 

 phenomenon" written from the point of 

 view of a microbiologist, he is referred to 

 Chapter 10 by Richards in Calkins and 

 Summers (1941). In the same volume, 

 Chapter 9 by Hall also rewards reading. 



Fixation of Poisons by Goldfish Popula- 

 tions. In addition to its relation to such 

 factors as reproduction and growth, condi- 

 tioning may also affect mortaUty. This it 

 may either increase, as when larval Tribo- 

 lium were grown in heavily conditioned 

 flour, or it may confer upon the total popu- 

 lation a lowered mortaUty by protecting 

 that population against some environmen- 

 tal vicissitude. This latter aspect, now to 

 be briefly developed, falls under our cate- 

 gory "fixation by the population of toxic 

 substances" and is well illustrated by the 

 study of AUee and Bowen (1932) on gold- 

 fishes. 



AUee and Bowen observed the survival 

 of isolated goldfishes compared with groups 

 when both were subjected to an environ- 

 mental poison, colloidal silver. Seven ex- 

 perimental units were set up, each consist- 

 ing of ten fishes in a hter of water plus a 

 colloidal silver suspension. The seven con- 

 trol units consisted of seventy isolated fishes 

 each in one liter of water and in colloidal 

 silver of the same strength and same sus- 

 pension as that to which the grouped fishes 

 were exposed. The life duration of both 

 control and experimental forms was re- 

 corded. It was found that the latter lived 

 on the mean 507 minutes, while the con- 

 trols lived only 182 minutes. The grouped 

 fishes thus had a mean fife duration ex- 

 ceeding that of the controls by 325 min- 

 utes, a significant statistical difference. 



In the experimental units the silver was 

 precipitated, while in the control units it 

 remained largely in suspension. When 

 exposed to the toxic colloidal silver, the 

 grouped fishes shared between them a 

 dosage easily fatal for any one fish. Appar- 

 ently the sfime secreted by the groups 

 changed much of the silver into a less 

 toxic form. As this experiment was con- 

 ducted, the silver suspension was some- 

 what too strong for any fish to survive, but 

 probably with a weaker suspension some or 

 all of the grouped animals would have 

 lived. Even as carried out, the group 



gained for its components a longer life, 

 and it seems likely that in nature it might 

 have had that many more minutes for, say, 

 rain to have diluted the water or for some 

 other disturbance to have cleared up the 

 poison and thus give the population an 

 opportunity for complete recovery. 



Conditioning of Water by Procerodes 

 Populations. Alice (1931) and Oesting and 

 Allee (1935) studied the mechanism b) 

 which the marine flatworm Procerodes 

 wheatlandi protects its populations through 

 conditioning from a hypotonic environ- 

 ment. These worms are abundant along 

 certain seashores where they live near 

 the low tidemark level or a little beyond. 

 There they frequently are found on the 

 undersurfaces of stones, perhaps as a pro- 

 tection from wave action, and usually they 

 are aggregated. 



A worm removed into the laboratory 

 and put into fresh water swells and begins 

 to cytofize. If the Procerodes are thor- 

 oughly washed to remove sea water from 

 their surfaces and then placed both as iso- 

 lated individuals and as groups into fresh 

 water, the latter survive for a definitely 

 longer time. The first worms to die in the 

 group do so about as soon as the first iso- 

 lated worms. However, as the dead worm 

 disintegrates, the habitat is altered, and, 

 because of this alteration or conditioning, 

 the remaining members of the group have 

 an increased fife expectancy beyond that 

 of isolated individuals. 



This can be extended. A sort of "worm 

 soup" is prepared by killing a number of 

 well-washed wonns and allowing them to 

 remain in the water in which they had 

 died and so condition it. Freshly collected 

 Procerodes five longer as individuals in 

 such conditioned medium than do similar 

 worms isolated into clean pond water. The 

 difference between the two media is only 

 that in one the worms had liberated prod- 

 ucts of disintegration, while in the other 

 such was not the case. This survival differ- 

 ential persisted even when the total amount 

 of salt in the two types of water was made 

 identical by adding dilute sea water to the 

 clean pond water. In such experiments the 

 worms still lived significantly longer in the 

 conditioned medium than did those in di- 

 lute sea water of the same salt concentra- 

 tion. 



When the Procerodes are in a hypotonic 

 environment their survival is increased il 



