XII.] CONJUGATION AND SEXUAL REPRODUCTION. 205 



of independent development is interfered with by the preHmi- 

 nary changes. But, just as eggs, in which these internal changes 

 have once been carried out, cannot remain indefinitely thus 

 prepared, but very soon change so that they are no longer 

 adapted for fertilization, and finally decay,— so it is with an 

 Infusorian which has passed the time for conjugation ; it be- 

 comes incapable of conjugating, and finally, of living. 



As far as I can see there is only one point of view from which 

 the gradual dissolution of an Infusorian which has not succeeded 

 in conjugating can be rightly regarded as a kind of natural 

 death ; viz. if we could prove that its destruction is dependent on 

 some adaptation especially directed to this end. Maupas is, naturally 

 enough, very far from accepting this point of view ; for he 

 clings to the old belief that death is a universal attribute of life, 

 and is not a phenomenon of adaptation. From my standpoint 

 we might argue as follows : — Conjugation must take place 

 periodically because the crossing of individuals is necessary for 

 the maintenance and development of the species. If it was im- 

 possible to ensure the occurrence of crossing in all or the great 

 majority of individuals and colonies, there would be a danger of 

 the uncrossed ones getting the upper hand. To prevent this, 

 the animals which do not conjugate must be prevented from 

 living on indefinitely, in fact natural death must occur, and this 

 was ensured by conferring upon the macronucleus of the 

 animal such a structure that it was used up during assimila- 

 tion, while the micronucleus was so constructed that it under- 

 went dissolution in consequence of the divisions preparatory 

 to amphimixis, or as we may otherwise imagine it. 



I know of no biological principles which are antagonistic 

 to such a view, but I scarcely believe that it is a correct one ; 

 analogy with the sexual cells is against it. I do not doubt that 

 nature would be quite capable of bringing about a natural death 

 for those animals which have escaped conjugation, if it were 

 necessary for the maintenance of the species ; but their destruc- 

 tion does not appear to be necessary. We should hardly 

 maintain that the dissolution of a spermatozoon which has 

 missed its mark is dependent on the appearance of natural 

 death, especially designed for it. On the contrary, it is ob- 

 viously destroyed simply because the vital conditions necessary 

 for its continued existence are wanting, viz. fusion with an 



