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SCIENCE 



[N. S. Vol. XXV. No. 636 



doubtedly becomes old, and after conjuga- 

 tion it apparently resumes its vigor. But 

 the latter process is more correctly inter- 

 preted as renewal instead of rejuven- 

 escence ; the old body is sloughed off a little 

 at a time and gradually renewed after con- 

 jugation from the germ-plasm, because it 

 can not be cast off an entire dying body 

 as in Metazoa. 



I believe that Weismann is correct in his 

 contention that the conception of rejuven- 

 escence has not a shadow of support among 

 the Metazoa, indeed would never have been 

 conceived from what we know about the 

 Metazoa themselves; and also that the con- 

 ception is baseless as applied to Protozoa. 

 The conception of germ-plasm and soma is 

 as necessary for Protozoa as Metazoa, and 

 the conception of senescence is unmeaning 

 as applied to germ-plasm. But Weis- 

 mann 's conclusion that the Protozoa are 

 potentially immortal does not follow; the 

 protozoan soma is no more immortal than 

 that of the Metazoa, and it is as little sub- 

 ject to rejuvenescence. 



No theory of sex can be consistent that 

 divorces the physiological significance from 

 the causes of sex-differentiation. In the 

 physiological significance, that is, in the na- 

 ture of the fertilization-need, we must find 

 the primary cause of sexual differentiation. 

 Richard Hertwig has been one of the very 

 few to recognize this axiomatic principle; 

 but he nevertheless states two hypotheses, 

 one of the physiological significance of fer- 

 tilization, the goal and ultimate attainment 

 of sex-differentiation, and the other of the 

 causes of sex-differentiation itself, and 

 these have no logical connection. The 

 main value of Geddes's and Thompson's 

 otherwise vague and unsatisfactory theory 

 of sex lies in their appreciation of the con- 

 nection between the physiological signif- 

 icance and the causes of sex-differentia- 

 tion. 



In Metazoa, fertilization is always select- 



ive, i. e., between unlike gametes. Ovum 

 does not fertilize ovum, nor do spermatozoa 

 conjugate. It is true that a phenomenon 

 known as fertilization by the second polar 

 globule has been described in a partheno- 

 genetic egg, but it is altogether improbable 

 that it has the physiological value of fertil- 

 ization. In Protozoa, also, fertilization is 

 often selective, i. e., between differentiated 

 gametes, and there are various degrees of 

 differentiation from conditions essentially 

 similar to the reproductive cells of Metazoa, 

 to relatively slight unlikeness of gametes; 

 and the latter grades into the conjugation 

 of like gametes, which seems to be the prim- 

 itive condition. It is almost universally 

 believed that selective fertilization does not 

 exist when the gametes are alike; any two 

 gametes may unite. It, therefore, follows 

 that the fertilization-need is the same in 

 both gametes (even when they are differen- 

 tiated) . And from this idea arises the in- 

 consistency between theories of the sig- 

 nificance and the causes of sex differentia- 

 tion; for if the gametes are in the same 

 physiological condition, their differentia- 

 tion, and sex differentiation itself, can only 

 be devices to secure gametic union. 



But there is an alternwtive point of view, 

 viz., that fertilization may be ahvays select- 

 ive, even when there is no morphological 

 gametic differentiation. I am convinced 

 that only on such an assumption can a con- 

 sistent theory of sex differentiation be con- 

 structed. If gametes be physiologically 

 different, even when they are morpholog- 

 ically alike, then morphological differentia- 

 tion of gametes follows naturally as an 

 expression of these physiological differ- 

 ences, and sex-differentiation as a further 

 stage in the same process of evolution. 



Now Calkins has clearly demonstrated 

 the probability that fertilization is select- 

 ive even when the gametes are moi*pholog- 

 ically alike. He showed that, in Parame- 

 cium, one of the ex-conjugants in each case 



