THEORY OF FERTILIZATION. 153 



force, on the liability of asexual, or what he calls unisexual multiplica- 

 tion to end in degeneration or extinction, and on the necessity of 

 double parentage for the preservation and progress of the species. 

 Similarly, Van Beneden, Biitschli, and Hensen have all spoken of 

 the process as a rejuvenescence (rejeunissement, Verjungung). The 

 asexual process of cell-muitiplication is limited; conjugation in lower, 

 fertilization in higher organisms supply the recurrent impulse which 

 keeps the life of the species young. According to Van Beneden, 

 ' ' The faculty which cells possess of multiplying by division is limked. 

 There comes a time when they can divide no further, unless they 

 undergo rejuvenescence by fertilization. In animals and plants, the 

 only cells capable of being rejuvenesced are the eggs ; the only cells 

 capable of rejuvenescing these are the sperms. All the other parts of 

 the individual are devoted to death. Fertilization is the condition of 

 the continuity of life. Par elle le generateur echappe a la mort." 

 Hensen, in his admirable "Physiology of Reproduction," expresses 

 the same when he says : "By normal fertilization, death is warded off 

 (ferngehalten) from the germ and its products." Biitschli has inter- 

 preted conjugation in similar terms. 



Weismann quotes the three opinions just mentioned, and vigorously 

 criticises them. He demands evidence for the limitation of asexual 

 reproduction assumed above, and • speaks of the "impossibility of 

 proof." The whole "conception of rejuvenescence has something 

 indefinite and misty about it." (Some may be obliged to plead 

 guilty to a similar impression in regard to Weismann' s keimplasma). 

 "How can one think that an infusorian, which by continued division 

 has at length exhausted its reproductive capacity, will regain the same 

 by uniting and fusing with another which has also lost its power of 

 further division ? Twice nothing can not give one ; or if one assumes 

 that in each animal there persists only half the reproductive capacity, 

 so that the two together would form one, this one can hardly call 

 'rejuvenescence.' It would be simply an addition, as is under other 

 circumstances attained by simple growth, — that is, if we leave out of 

 account what in my eyes is the most important moment in conjugation, 

 namely, the mingling of two heredity-tendencies ( Vererbimg- 

 stendenzeti) ." Does Professor Weismann not feel that there is some- 

 thing "indefinite and misty" even about this? He sarcastically 

 compares the two exhausted individuals to two exhausted rockets, 

 which are supposed to rejuvenesce in mutually affording the 

 constitutents of nitroglycerine. More forcibly he urges the difficulty 

 suggested by continued parthenogenesis, — a difficulty which we shall 

 afterwards have to discuss. "To the conception of rejuvenescence," 

 he says, in conclusion, "I could only agree, if it were proved that 



