Heredity in Protozoa 62 1 



hours; among these there were eight groups of united individ- 

 uals — some of two, some of several, specimens united in chains. 

 The eight groups were again isolated (June 25). 



Effects of Natural Selection 



These eight groups showed many imperfect individuals, and 

 the groups were at a great disadvantage as compared with the free 

 individuals. This was because they are not able to swim about 

 actively, but must lie at the bottom and move about only irregu- 

 larly. As a result they get comparatively little food, and are not 

 able to avoid regions where the conditions are harmful. The bac- 

 teria multiplied much more rapidly than in the free culture, con- 

 taining many individuals — the latter keeping down the number 

 of bacteria by feeding on them. 



In consequence of these bad conditions, the united groups began 

 to die. Some multiplied farther, all the individuals remaining 

 united. But forty-eight hours after the isolation of the second lot 

 of eight groups, all were dead. 



Thus it is easy to produce by selection a culture containing only 

 free individuals and multiplying in the usual way. Artificial 

 selection will likewise produce a culture of united specimens, 

 multiplying mainly by incomplete fission. But at the same time 

 natural selection acts; these groups die, owing to their inefficiency 

 in getting food, keeping down the bacteria, avoiding harm, and 

 in the performance of the general bodily functions. 



This extinction by natural selection of the series multiplying 

 by incomplete fission was shown in another way. A considerable 

 number of the progeny of a, with both separated individuals and 

 united groups, was allowed to accumulate in a shallow watch glass. 

 Here the united groups flourished fairly well, because the vessel 

 was so shallow that they received plenty of oxygen and of food 

 while lying on the bottom, while the undue multiplication of the 

 bacteria was prevented by the numerous free individuals. Now 

 the culture was transferred to a large vessel, three inches deep. 

 Here the culture multiplied enormously, but all the groups of 

 united specimens quickly disappeared. They sank to the bottom 



