670 LECTURE XXIII. 



such as to elude any methods of investigation. Still these 

 facts have their significance. 



The main objection to the view of Strasburger and Weis- 

 mann is that it fails to afford any explanation of the phenomena 

 of sexual reproduction. If male and female gametes are 

 essentially alike, why is it that two oospheres or two an- 

 therozoids never coalesce, but only antherozoid with oosphere, 

 and how are the manifestations of sexual affinity, as we find 

 them in the case of Acetabularia, Ectocarpus, Dasycladus 

 (p. 647), and in the Phanerogams (p. 648) to be accounted for ? 



Strasburger and Weismann attempt to meet this objection 

 by reference to Pfeffer's observation (see Lect. XX. p. 529) 

 that, in cases in which the sexual cells are well differentiated, 

 the female organ excretes a substance which has an attracting 

 influence, and that it is only the corresponding male cells 

 which are susceptible to this influence. The reason why two 

 antherozoids or two oospheres do not coalesce is then this, 

 that they do not attract each other ; and it is for the same 

 reason that a coalescence of gametes of opposite sexes, but 

 not of the right degree of sexual affinity, does not take place. 

 But the facts observed by Pfeffer do not really touch the 

 point at issue. The attractive substances in question serve, it 

 is true, to bring the motile male gametes, which may be pro- 

 duced at a distance, into proximity with the female gamete, 

 but there is no evidence that they determine the actual 

 coalescence of the two gametes. Pfeffer's observations do 

 not account for the sexual process. 



We cannot but conclude that the facts of sexual repro- 

 duction cannot be satisfactorily accounted for, otherwise than 

 on the assumption that male and female gametes are es- 

 sentially diverse. It is in fact because they are diverse that 

 they are male and female : it is just this essential diversity 

 that constitutes sex. External differences do not constitute 

 sex ; they are adaptive differences of only secondary im- 

 portance. We assume that even in those plants which have 

 externally similar gametes, there is yet a sexual difference 

 between them. It is easy to conceive that objection may be 

 taken to this view ; but it appears to be quite necessary to 



