1 yo THE EVOLUTION OF SEX. 



Meanwhile Schaffer had observed the occurrence of partheno- 

 genesis in minute aquatic crustaceans, the study of which has 

 since shed some vivid light on the v/hole subject. Pastor 

 Dzierzon had also clipped the wings of queen -bees, and in 

 thus preventing their nuptial flight and impregnation, observed 

 that the eggs they laid developed only into drones. The facts 

 soon began to be recognised, extended, and thought over by 

 naturalists of the standing of Owen (1843), Von Siebold (1856), 

 and Leuckart (1858), whose conclusions have afforded a firm 

 basis for the abundant subsequent observation and speculation 

 on this interesting subject. 



§ 2. Degrees of Parthe?iogenesis. — If we start then with Von 

 Siebold's definition of parthenogenesis, as the power possessed 

 by certain female animals of producing offspring without sexual 

 union with a male, it will clear the ground to notice, in the first 

 place, the numerous different degrees in which this development 

 without fertilisation may occur. 



{a) Artificial Parthenogenesis. — There are a few curious 

 observations which go to show that in exceptional circumstances 

 ova may develop when the male stimulus is replaced by some 

 artificial reagent. These observations must still be taken ami 

 grano salis, but they may be at least suggestive of further 

 experiment. Dewitz observed unfertilised frog ova to undergo 

 segmentation {sic) in corrosive sublimate solution. In some 

 cases one division occurred, in others several ; in some cases 

 irregularly, in others normally. It happened both when the 

 ova were left in the reagent, and when they were merely dipped 

 and returned to water. The eggs experimented on were those 

 of the two common frogs Rana fusca and R. esculenta^ and of 

 the tree-frog {Hyla arborea). But it must be noted that Leuckart 

 long ago noted the occurrence of spontaneous division in 

 frog ova. Similarly, Tichomiroff, experimenting with the un- 

 fertilised ova of the silkmoth, which are occasionally partheno- 

 genetic, was surprised to observe that ova, which would not of 

 themselves develop parthenogenetically, might be induced to 

 do so by certain stimuli. These consisted in rubbing the 

 unfertilised ova with a brush, or in dipping them for two minutes 

 in sulphuric acid and then washing them. In both cases, he 

 says, a percentage of the ova thus artificially stimulated de- 

 veloped. It must be remembered that occasional partheno- 

 genesis occurs in this insect, and all that Tichomiroff did was 

 to incite this. There is no doubt that reagents may con- 



