XII.] CONJUGATION AND SEXUAL REPRODUCTION. 201 



nuclei should be in the state best adapted for fusion, — viz. 

 that the mass should be diminished so far as to reduce the 

 hereditary units, or ids, to half. And all this as a matter of 

 fact takes place. But it is nevertheless insufficient to ensure 

 the desired result ; for Maupas' experiments show us that, in 

 spite of it, conjugation may be absent. The impulses which 

 induce Infusoria to seek one another, and to pair, appear 

 at certain periods and under certain external conditions, but 

 if the latter are not favourable the impulses are not manifested 

 and after the lapse of some time the power of conjugation 

 is completely lost in the colony in question. I assume that 

 Maupas' observations are correct, and am not criticizing them ; 

 but his own results prove, in my opinion, that his interpre- 

 tations are erroneous in so far as he endeavours to find 

 support for the theory of rejuvenescence by means of the facts 

 which he has observed. Those colonies which have passed 

 the proper time for conjugation gradually die out. Maupas 

 considers that they die a ' naturaP death in consequence of 

 old age. He claims to have proved the occurrence of ' physio- 

 logical' death in unicellular organisms, and to have refuted 

 my views as to their potential immortality. 



But I believe that the facts brought forward by him are 

 capable of a different and a more correct interpretation. 



What happens when a colony has passed the appropriate 

 period and has therefore lost the power of conjugation ? The 

 very same thing which happens to the ovum which has attained 

 maturity and has extruded its polar bodies — disintegration, 

 preceded by the loss of all power of development. I believe that 

 this result proceeds from the same cause in both cases, — 

 the reduction of nuclear substance, i. e. in the Infusorian, the 

 substance of the micronucleus. The egg disintegrates because 

 the nuclear substance is insufficient for the commencement of 

 ontogeny, and is imperfectly adapted for its preservation ; 

 the Infusorian disintegrates because its macronucleus must be 

 renewed periodically, and because this cannot occur after the 

 micronucleus has perished. And Maupas informs us that the 

 latter disintegrates sooner or later, if the proper time for 

 conjugation has passed by. 



If we ask, how is it that the micronucleus disappears when 

 conjugation is excluded, Maupas answers that the necessary 



