164 THE EVOLUTION OF SEX. 



admirable "Physiology of Reproduction," expresses the same 

 when he says : — " By normal fertilisation, death is warded off 

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

 interpreted 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 cannot give one ; or if one 

 assumes that in each animal there persists only half the repro- 

 ductive 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, viz., the mingling 

 of two heredity-tendencies {^Vererbimgstetidenzeii).'''' (Does Pro- 

 fessor Weismann not feel that there is something " 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 constituents 

 of nitroglycerine. More forcibly he urges the difficulty sug- 

 gested by continued parthenogenesis, — a difficulty which we shall 

 afterwards have to discuss. " To the conception of rejuvenes- 

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

 proved that multiplication by division can never, — not merely in 

 certain conditions, — but never continue unlimitedly. This 

 cannot, however, be proved, just as little as the reverse." But 

 Weismann must surely admit, that the demonstration of even 

 some cases where species, normally reproducing asexually, come 

 to an absolute standstill if conjugation be prevented, would 

 give considerable strength to the interpretation of fertilisation 

 as rejuvenescence. Such cases have, happily, come to hand, as 

 we shall now see. 



We have already referred to Maupas's proof of true sexual 

 union in ciliated infusorians. By an elaborate process of 

 nuclear division, disruption, elimination, interchange, union, 



