194 MANUAL OF ELEMENTALLY ZOOLOGY 



asexually. 1 In examining the meaning of conjugation we 

 shall be justified, by the very fact that it takes place 

 in spite of the lessening in numbers which it causes, and 

 also by the elaborate preparations which are often made 

 for it, in assuming that it is in some way beneficial, and 

 not merely brought about by certain circumstances without 

 regard to the welfare of the being in which it occurs. The 

 problem is to discover what is the benefit which it confers 

 upon the organism. With regard to this several theories 

 have been held. 



(i) On account of the fact that the conjugation of the 

 germs of higher animals is followed by the rapid nuclear 

 division which leads to the cell formation of development, 

 it has been supposed that the process is primarily a stimulus 

 to nuclear division, which is needed in certain cases. But 

 if that were so the nuclear division which leads to fission in 

 Paramecium ought also to be quickened by conjugation, 

 whereas we find here that conjugation takes place after a 

 period of frequent divisions and is followed by a slackening 

 in the rate of division — unless, indeed, a new period of 

 plenty ensue, in which case it is the food supply, not con- 

 jugation, which is responsible for the quickening of fission. 

 Conjugation must meet some need other than the stimula- 

 tion of division. In the higher animals it comes, indeed, 

 to act as a stimulus to division, but that is incidental. 

 In these animals the whole body cannot conjugate, but 

 their germs consist each of a single energid and are thus 

 able to undergo conjugation. The germs are, moreover, 

 incapable of passing out of their simple condition by 

 development until they have first conjugated, and this 

 fact prevents them from missing conjugation and thus 

 losing its benefits, whatever these may be. Conjugation 

 over, however, they must undergo division to form the 

 adult body. That this happens is ensured by the fact 

 that conjugation imparts to them a stimulus to division, but 

 this stimulus is not inherent in conjugation ; it is added to 



1 In the extreme case of Paramecium, where each parent forms only 

 two gametes, the halving of numbers by conjugation brings it about 

 that the zygotes are no more numerous than the parents, so that here 

 sexual reproduction produces new individuals without increasing the 

 total number, which grows only by asexual fission. 



