178 THE EVOLUTION OF SEX. 



for days producing a viviparous brood, at the rate of one per 

 hour; the offspring soon begin themselves to multiply ; and 

 Huxley calculates, that if this continued for a year without 

 mortality, a single aphis would be the ancester of a progeny 

 which would weigh down five hundred millions of stout men ! 

 Not gardeners only have cause for gratitude that climate and 

 enemies prevent such untoward increase. But there are other 

 desiderata besides numbers. Can it be said that parthenogenesis 

 favours the general life and progress of the species? It will 

 be at once recognised that rotifers, brine-shrimps, water-fleas, 

 aphides, coccus-insects, and so on, are relatively low forms. 

 Only two or three butterflies and one beetle are parthenogenetic. 

 Higher up in the scale virgin birth never occurs except in a 

 very partial and pathological degree. But we can go further. 

 More than one of the old naturalists, and in recent years 

 Brooks, Galton, Weismann, and others, have laid emphasis on 

 the value of fertilisation as a fountain of change. To Weismann 

 the intermingling of the male and female " germ-plasmas " in 

 fertilisation is really the sole source of variation. That it is a 

 source, all will admit. If it be removed therefore, as in rotifers, 

 the species will be so much the less likely to progress. Weis- 

 mann holds that it will not progress at all ; and though we 

 should not go quite so far, we are bound to allow that the 

 establishment of parthenogenesis is a handicapping of evolution. 

 We cannot, however, follow Weismann in his next step. If 

 all change springs from the sexual intermingling, the rotifer 

 species cannot change at all. They cannot go forwards, nor 

 yet backwards. Having attained to a physiological state when 

 males became superfluous, they remain 171 statu quo. So he 

 emphasises that superfluous organs, such as the sperm-receptacle, 

 do not become rudimentary in parthenogenetic species, — " rudi- 

 mentary organs can only occur in species with sexual reproduc- 

 tion." This is a corollary of Weismann's contention that no 

 individually acquired characters, either plus or minus, can be 

 transmitted, and that the sexual intermingling is the sole source 

 of change affecting the species. Were the main propositions 

 proven, the corollary would follow, but there are still many 

 dissentient voices. Without going into the general question 

 at present, let us take the corollary by itself, (i.) Cases where 

 males are quite unknown are comparatively few ; in most 

 cases they reai)pear at intervals. It is not possible, therefore, 

 as Weismann will allow, to be certain that the sperm-receptacle 



