XI.] PROBLEMS OF THE DAY. 95 



possibly incomplete, and in order to expose the correctness of 

 my view to the widest available test, I carried out its logical 

 consequences as thoroughly as possible, and laid down the 

 principle that species which propagate parthenogenetically 

 have no power to develop into new species. Furthermore, 

 about the same time, I began a series of experiments to test 

 the truth of this statement as to the capacity for variation pos- 

 sessed by parthenogenetic species ; these have been continued 

 up to the present time, and on some future occasion I hope to 

 make them public. 



But even if, as seems at present very probable, sexual repro- 

 duction is not the only origin of individual variability in the 

 Metazoa, no one will deny that it is the chief means of increas- 

 ing these variations and of continuing them in favourable pro- 

 portions. In my opinion, the importance of the role which 

 sexual reproduction plays in shaping the material for the 

 process of selection is scarcely diminished, even if we con- 

 cede that some amount of individual variability can be called 

 forth by direct influences on the germ-plasm. Even Professor 

 Vines considers it probable 'that the absence of sexuality in 

 these plants (the parthenogenetic higher Fungi) may be just the 

 reason why no higher forms have been evolved from them ; for 

 in this respect they present a striking contrast to the higher 

 Algae in which sexuality is well marked '.' 



But when Professor Vines says ' there can be no doubt that 

 sexual reproduction does very materially promote variation -.' 

 he does not intend to imply that this statement is self-evident : 

 for it is well known to him that prominent investigators, like 

 Strasburger^, see in sexual reproduction the reverse action, 

 ' that of preserving the constancy of specific characters.' I accept 

 with pleasure his agreement with my view, confirming the 

 chief result of my fifth essay, which may be expressed as 

 follows : — Sexual reproduction has arisen by and for natural 

 selection, as the only means by which the individual variations 

 can be united and combined in every possible proportion. 



Again, with respect to the problem of the inheritance of ac- 



1 ' Nature,' Oct. 1889, p. 626. ^ < Nature,' Oct. 1889, p. 626. 



•' Strasburger, ' Neue Untersuchungen fiber den BelVuchtungsvorgang 

 bei den Phanerogamen als Grundlage fiir eine Theorie der Zeugung.' 

 Jena 1884, p. 140. 



