DEGENERATE SEXUAL REPRODUCTION. 171 



siderably modify ova ; thus the brothers Hertwig showed how it 

 was in this way possible to overcome the non-receptivity of the 

 ovum to more than one sperm. Nor can one forget how sexual 

 reproduction in parasitic fungi tends to disappear, being ap- 

 parently replaced by the stimulus afforded from the waste pro- 

 ducts of the host. In a similar way, the multiplication of 

 cells, so frequently associated in disease with the presence of 

 bacteria, has been referred by more than one pathologist to the 

 " spermatic influence " of these micro-organisms, or of the 

 katastates which they form. 



{^) Pathological Parthenogenesis. — It has very occasionally 

 been noticed in higher animals, where true parthenogenesis is 

 wholly unknown, that an unfertilised egg starts off on its own 

 resources without any male stimulus whatever. This is noted 

 by I>euckart 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 patho- 

 logical formations which not unfrequently 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 Greeff, who saw unfertilised ova of the common 

 starfish developing in ordinary sea water, in a perfectly normal 

 fashion, only more slowly than usual. 



ic) 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 successful, often in fact reaching 

 maturity, and also in this, that since related forms are partheno- 

 genetic, 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," Weismann says, 

 " reproduce exceptionally by parthenogenesis, for instance many 

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

 an unfertilised female lays develop, but only a fraction, and 

 usually a very small fraction of the total number, the rest perish- 

 ing. 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 Pai'thenogenesis. — The queen-bee, as has been 

 already mentioned, is impregnated by a drone in her nuptial 



