^6 THE EVOLUTION OF SEX. 



VI. Effects of Parthenogenesis. — Since parthenogenesis is 

 dominant in rotifers, and well-established among water-fleas and plant- 

 lice, it is very plain that whatever else it affects, it is anything but 

 prejudicial to numbers. An aphis will continue 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 ancestor 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 favors the general life 

 and progress of the species? It will be at once recognized 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 fertilization as a fountain of change. To Weismann the inter- 

 mingling of the male and female ' ' germ-plasmas ' ' in fertilization is 

 really the 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. Weismann 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 evo- 

 lution. 



We can not, however, follow Weismann in his next step. If all 

 change springs from the sexual intermingling, the rotifer species can 

 not change at all. They can not go forward, nor yet backward. 

 Having attained to a physiological state when males became super- 

 fluous, they remain in statu quo. So he emphasizes that superfluous 

 organs, such as the sperm-receptacle, do not become rudimentary in 

 parthenogenetic species, — "rudimentary organs can only occur in 

 species with sexual reproduction." 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 propo- 

 sitions 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 reappear at 

 intervals. It is not possible, therefore, as Weismann will allow, to be 

 certain that the sperm-receptacle becomes superfluous to the species. 



