138 J. A. DAWSON 



the animals did not divide by fission, but became smaller, darker 

 in appearance, and more sluggish in movement. Specimens 

 stained during this period had a normal structure when examined 

 cytologically, the decrease in size being the only noteworthy 

 feature. 



Attempts have been made to keep these twins alive by means 

 of mass culture similar to those used to keep alive representatives 

 of culture A, but it has invariably been found that they soon dis- 

 appear from such cultures — the maximum time before their total 

 disappearance being three weeks — although the single animals 

 derived from the twins multiply and live as long as conditions 

 are kept favorable. 



The elimination of twins in mass cultures is probably due to 

 the combined operation of several factors. In the course of fission 

 a twin frequently divides giving one twin and two single animals. 

 In a few generations it will be readily seen that twin animals tend 

 to be outnumbered by single animals. The only instance in 

 which a twin shows greater activity than a single animal is in its 

 rapid rotation while swimming. At all other times the single 

 animal moves more actively, which undoubtedly aids in its se- 

 curing a better food supply. Possibly, owing to the fact that 

 the single animal has a greater surface per unit volume, it is 

 better able to carry on metabolic processes. The survival of 

 single animals recalls the observation by Jennings ('08) in which 

 he found that united specimens of paramecia in mass cultures 

 soon disappeared. This he attributed to their more sluggish 

 movements and consequent inability to obtain an adequate food 

 supply in competition with the more active single animals. 



b. Effect of selection on 'twins' {series I) 



When the original twin the progeny of which formed culture 

 AT was isolated, a series of experiments was begun using animals 

 from the twin culture (AT) in order to find out if a strain could 

 be obtained which would breed true for twins. Cultures were 

 carried of animals which gave 100 per cent of twins for three or 

 four generations. As the number of such cultures which could be 



