1 66 THE EVOLUTION OF SEX. 



Without the normal sexual union, then, the family becomes 

 senile. Powers of nutrition, division, and conjugation with 

 unrelated forms, come to a standstill. This senile degeneration 

 is very interesting. The first symptom is decrease in size, 

 which may go on till the individuals only measure a quarter of 

 their normal proportions. Various internal structures then 

 follow suit, "until at last we get formless abortions, incapable of 

 living and reproducing themselves." The nuclear changes are 

 no less momentous. The important para- or micro -nucleus 

 may partially or completely atrophy, and conjugation is thus 

 fatally sterile. The larger nucleus may also become affected, 

 " the chromatin gradually disappearing altogether." Physiolo- 

 gically too, the organisms become manifestly weaker, though 

 there is what the author calls a " surexcitation sexuelle." Such 

 senile decay of the individuals and of the isolated family in- 

 evitably ends in death. 



The general result is evident. Sexual union in those infus- 

 orians, dangerous perhaps for the individual life, — a loss of time 

 so far as immediate multiplication is concerned, — is in a new 

 sense necessary for the species. The life runs in cycles of 

 asexual division, which are strictly limited. Conjugation with 

 unrelated forms must occur, else the whole life ebbs. ^Vithout 

 it, the Protozoa, which some have called " immortal," die a 

 natural death. Conjugation is the necessary condition of their 

 eternal youth and immortality. Even at this low level, only 

 through the fire of love can the phoenix of the species renew 

 its youth. 



At the beginning of this century, the too-much-forgotten 

 biologist Treviranus directed attention to fertilisation as a 

 source of variation, and his suggestion has been several times 

 independently revised. 



Thus Brooks, to whose works we have repeatedly referred, 

 has emphasised not only the importance of fertilisation as a source 

 of progressive change, but further, that the male element is 

 much the more important in this connection. 



Similarly, though on somewhat different lines, Weismann 

 finds in the mingling of male and female Keimplasmas the 

 source of those variations on which natural selection operates. 

 Rejecting as he does the alleged inheritance of acquired 

 characters, he finds the fountain of change in sexual repro- 

 duction. " Sexual reproduction is well known to consist in the 

 fusion of two contrasted reproductive cells, or perhaps even in the 



