Studies on the Life History of Protozoa. 447 



It is not out of place to consider first the initial objects of the 

 undertaking, although at the risk of again repeating what has been 

 often stated. 



1. The first aim of the experiments was to get light upon the 

 general phenomenon of conjugation and through this, upon fertil- 

 ization in general. 



2. To determine whether conjugation is imperatively necessary 

 for rejuvenescence. 



3. To determine whether artificial rejuvenescence is possible. 



4. To determine the conditions, antecedent and subsequent to 

 conjugation. 



5. To determine, if possible, the significance of rejuvenescence. 



6. To determine, finally, whether protoplasm in these simple 

 forms is capable of indefinitely continued life without conjugation, 

 or whether it is subject to the conditions of "old age." 



On none of these points can a definitely positive answer be 

 given, and further experiments must be undertaken to clear them 

 up. The fact that, after a continuous cultivation of 742 genera- 

 tions, covering a period of 23 months, the race died out apparently 

 from exhaustion, shows that under the conditions, continued life 

 was impossible, and if this conclusion, which seems to be the only 

 one justified by the results, be granted to obtain in nature, then 

 we must agree with Maupas and others that the indefinite con- 

 tinuance of life without conjugation, is improbable. 



I. The Conditions of the Experiments. 



The question has been raised whether the conditions under 

 which the experiments were undertaken were in any way abnor- 

 mal to Paranicecium, and whether, from the results obtained, we 

 are justified in carrying the conclusions to the free-living forms, 

 and to similar types in general. 



It might be objected that the space allowed was inadequate; 

 or, that the light conditions were abnormal; or, that the water 

 would get foul; or, that they were given only one kind of food; 

 or, that they were subjected to pressure. If we examine these 

 objections critically we shall find that they have little basis. 



Let us consider first the matter of space, for this involves some 

 of the other objections, viz: pressure, volume conditions, and the 



