RELATIONS OF OVA AND GEMMAE. 207 



from analogy, being put down to the account of imperfect 

 observation. 



The great difficulty, indeed, in the way of admitting a 

 total and continued absence of the sexual act in any species 

 is its apparent contrariety to the general analogy of the 

 organic creation. The constancy of the presence of both 

 sexes in almost every species, and the remarkable adapta- 

 tions which are provided in particular cases to ensure im- 

 pregnation, certainly suggest the idea that the act is one of 

 essential importance to the perpetuation of race. The act 

 may take place with less frequency in certain cases in- 

 deed it is well known that in some species the occurrence of 

 males is rare and intermittent but still it is one which we 

 have difficulty in conceiving ever to be wholly dispensed 

 with.* 



Now the cases of Parthenogenesis, above referred to, do 

 not appear materially to affect this conclusion. They still 

 leave the question open of the necessity of an act of sexual 

 reproduction recurring from time to time in the continued 

 propagation of a species. In the case of the bees it is 

 clear that the act of monogenesis must alternate with one 

 of a sexual kind, as the unimpregnated ova produce only 

 male insects. In all instances, indeed, yet observed, agamic 

 broods have been always of one sex in the same species, a 

 circumstance which is in favour of the occasional recurrence 

 of the sexual act, as the probable determining cause of the 

 development of the other sex.'f 



* Professor Braun of Berlin, in the Transactions of the Royal Prussian 

 Academy (Annals of Nat. Hist., 2d Ser., XVI., 234), mentions as a some- 

 what parallel case to the intermittent recurrence of male Entomostraca, 

 the casual appearance of stammiferous flowers in the weeping willow 

 (Salix Babylonica) , which has always been propagated by cuttings de- 

 rived originally from one female plant, and which ordinarily bears only 

 pistilliferous catkins. 



f This may hold of normal agamic broods, but Siebold found that when 

 the unimpregnated eggs of the silkworm-moth proved fertile, the resulting 



