^o THE EVOLUTION OE SEX. 



{b) Pathological Parthenogenesis. — It has very occasionally been 

 noticed in higher animals, where true parthenogenesis is wholly 

 unknown, that an unfertilized egg starts off on its own resources 

 without any male stimulus whatever. This is noted by Leuckart for 

 frog-ova, by Oellacher for hens' eggs, and by Bischoff and Hensen 

 even in mammals. Such cases must be regarded as rare abnormalities, 

 comparable perhaps to parthological formations which not unfre- 

 quently take place in the ovary, and it is hardly necessary to say 

 that in no case did the development proceed far. Balfour has also 

 cited a remarkable observation of Greff, who saw unfertilized ova 

 of the common starfish developing in ordinary sea- water, in a perfectly 

 normal fashion, only more slowly than usual. 



(c) Occasional Parthenogenesis. — In some of the lower animals, 

 which are not themselves normally parthenogenetic, but have relatives 

 so addicted, occasional parthenogenesis has been frequently observed. 

 These differ from the above cases, since the results are more suc- 

 cessful, often in fact reaching maturity, and also in this, that since 

 related forms are parthenogenetic, the ' ' abnormality ' ' is evidently 

 of a much milder type. The common silkmoth is a good example of 

 this occasional parthenogenesis, which certainly occurs, though rare 

 both in the genus and family. " A whole series in insects," Weis- 

 mann says, ' ' reproduce exceptionally by parthenogenesis, for instance 

 many butterflies, but that never to the extent that all the eggs which 

 an unfertilized female lays develop, but only a fraction, and usually 

 a very small fraction of the total number, the rest perishing. 

 Examples of successful occasional parthenogenesis (to the extent 

 at least of producing males) are furnished by those worker bees, 

 wasps, and ants which exceptionally become fertile." 



{d) Partial Parthenogenesis. — The queen-bee, as has been already 

 mentioned, is impregnated by a drone in her nuptial flight. The 

 sperms thus received are stored up, and used to fertilize the eggs as 

 she lays them in the cells. Not all the eggs, however, but only those 

 which will produce future queens or else workers. Other eggs, to all 

 appearance similar, are unfertilized, and these, as Dzierzon first 

 clearly showed, develop solely into drones. We can not, however, 

 say that the absence or presence of fertilization is the sole difference, 

 though if fertilization be prevented by the imperfect development of 

 the wings, or by clipping them, the queen only lays drone-eggs. The 

 same happens when she is old and her store of male elements 

 exhausted, or when the sperm receptacle has been removed. Von 

 Siebokl carefully examined the eggs from drone-cells, and found 

 that they never contained spermatozoa. Hensen notes an interesting 

 side fact, obviously corroboratory, that "German queen-bees, ferti- 



