CAUSES OF GENETIC VARIATION 217 



acquiring the kinetonucleus, Werbitzki states that in one case 

 passage through 50 animals and treatment with dyes left the 

 strain unaltered; but that in another case at the sixteenth 

 passage 7 per cent, of the trypanosomes were found to have 

 re-acquired the organ, and in subsequent passages the percentage 

 increased, until at the twenty-seventh passage practically all had 

 re-acquired it. Kudicke, however, in similar experiments did not 

 succeed in causing re-acquisition by transplantation. 



By the action of various drugs and antibodies races of try- 

 panosomes resistant to those substances have been obtained. 

 These breed true, at least when kept in the same species of animal 

 in which the resistance was acquired. As to whether change of 

 virulence is produced by passage through certain animals or not, 

 there is as yet no general agreement. 



Other changes, especially in size and some points of structure, 

 are said to occur when certain trypanosomes proper to mammals 

 are passed through cold-blooded vertebrates (Wendelstadt and 

 Fellmer), and it is stated that these changes persist, but the 

 observations have not yet been confirmed. 



Experiments lately conducted by Woltereck with Daphnia 

 are interesting as having given a definite positive result, in so far, 

 at least, as the ova were affected by conditions before leaving the 

 bodies of the parent individuals. The observations relate to 

 the offspring resulting from par theno genetic eggs. Females 

 bearing ephippia (fertilised eggs) were isolated until the ephippia 

 were dropped, and in this way the offspring of fertilisation were 

 excluded. Males, of course, appeared from time to time in the 

 cultures, but as fertilised eggs were rejected, their presence did 

 not disturb the result. The most remarkable observations 

 related to Daphnia longispina. 



This species as found in the lower lake at Lunz had the front 

 end of the body blunt and nearly round in profile; but on being 

 cultivated in a warm temperature and with abundant nourish- 

 ment the front end of the body became produced into an elon- 

 gated "helmet," as Woltereck calls it. Experiment showed that 

 the change was primarily due to the abundance of food, and owing 

 to temperature in a subordinate degree. 



