THEORY OF FERTILIZATION. 155 



ferent stages were again observed to conjugate successfully with unre- 

 lated forms, and this was done on Ho the one hundred and thirtieth 

 generation. After that, however, the family being again near its end, 

 the removal was no longer of any use. About the one hundred and 

 eightieth generation, the strange sight was seen of individuals of 

 the same family attempting to unite with one another. The results 

 were, however, nil, and the conjugates did not even recover from 

 the effects of their forlorn hope. 



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 conjuga- 

 tion is thus fatally sterile. The larger nucleus may also become 

 affected, " the chromatin gradually disappearing altogether." Physio- 

 logically 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 inevitably ends 

 in death. 



The general result is evident. Sexual union in those infusorians, 

 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. Without it, the Protozoa, which some have 

 called "immortal," die a natural death. Conjugation is the neces- 

 sary 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 fertilization as a source of variation, 

 and his suggestion has been several times independently revised. 



Thus Brooks, to whose works we have repeatedly referred, has 

 emphasized not only the importance of fertilization 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 



