Races of Paramecium. 559 



race into two, or transformation of one race into another. The 

 fact that no cases of this have occurred in my three j^ears' work 

 where many races existed side by side in the same laboratory, 

 must weigh heavily in interpreting any cases in which one race 

 apparently gives rise to another. 



The precautions required against admixture are mainly those 

 for preventing fluid containing individuals of one race from 

 getting into a culture containing another. Pipettes used in 

 handling the animals should invariably be thoroughly rinsed in 

 hot water immediately after they are used; for this purpose 

 it is necessar}' to have at hand at all times daring work a vessel 

 of water over the gas flame. Neglect of this precaution is fatal. 

 All fluid added to the cultures should be heated beforehand to at 

 least 50 degrees Centigrade to destroy any Paramecia it con- 

 tains. It is important not to make the mistake of supposing 

 that Paramecia can be filtered out by the use of filter paper; a 

 trial of this on a rich culture of Paramecium, with subsequent 

 microscopic examination of the fluid w^hich comes through will 

 convince anyone of the mistake of this procedure. 



On the other hand, my experience does not indicate that there 

 is any danger of transferring the animals dry, either in the air, 

 as dust, or attached to solids. Dry hay added to water contain- 

 ing no Paramecia never gives a culture of Paramecium. Adding 

 dry hay to a culture of a single race never results in an admixture 

 of races. Leaving open a culture vessel containing a single uni- 

 form race, even in a laboratory containing dozens of cultures 

 of other races, never results in an admixture of races, provided 

 that no fluid is allowed to drop into the open culture. On all 

 these points my experience has been extensive and long continued, 

 with absolutely concordant results. There is no experimental 

 evidence from any source that Paramecium may be dried, and 

 later revive when introduced into water. 



Thus there is great danger in the introduction of fluids into the 

 culture; little or none in the addition of dry substances. 



