INTRODUCTORY 



5 



is characteristic of all the higher plants and higher animals, some 

 of which, however, also reproduce asexiially. It is important to 

 note that even among unicellular organisms whose ordinary 

 method of multiplication is simple binary fission there is some- 

 times a distinct process of a sexual kind consisting of the union 

 of two individuals, and this process is known as conjugation. 

 The union may be either permanent or temporary, and when it 

 is temporary there is an interchange of nuclear material carried 

 out during the process. In each case the act is usually followed 

 sooner or later by fi,ssion. 



There is general evidence that conjugation has a rejuvenating 

 influence and that the individuals concerned are better fitted 

 thereby to perpetuate the race, but the problem as to the precise 

 signification of the process is still obscure. Maupas was the first 

 to deal with the problem experimentally, and his observations 

 upon the conditions under which conjugation took place in the 

 infusorian, Stylonichia, have become classical, though his con- 

 clusions can no longer be accepted in their entirety. Maupas 

 found that Stylonichia could go on giving rise to new individuals 

 by sim})le binary fission without any conjugating process taking 

 place for 215 generations, but that at this stage the individuals 

 produced were debilitated and no longer possessed the normal, 

 power of acquiring nutrition, and, moreover, the capacity to 

 divide was lost, so that the strain died out. If, however, 

 individuals were allowed to conjugate with other unrelated ones 

 of the same species before the stage of exhaustion was reached, 

 or at any time up to the 130th division, they became rejuvenated 

 and multiplication by binary fission could go on as before. After 

 the stage of exhaustion or debilitation had been reached it was 

 difficult or impossible to induce conjugation. From such experi- 

 ments Maupas drew the conclusion that there is a limit to the 

 capacity to reproduce asexually,^ that is, without the occurrence 

 of conjugation, and that the latter process, whereby a fusion of 

 protoplasm from two different individuals takes place, is essential 

 for the rejuvenation of the race, which cannot otherwise be 

 perpetuated indefinitely. 



Subsequent investigators, however, have found that in certain 

 Protozoa multiplication by binary fission may go on apparently 

 quite indefinitely without the intervention of conjugation at 

 all. Thus Calkins stated that with the slipper animalcule or 



