XIT.] CONJUGATION AND SEXUAL REPRODUCTION. IO9 



that we must ' not, on account of these isolated cases, under- 

 estimate the fact that the necessity for fertihzation is predomi- 

 nant, and controls, to their most secret depths, the sources of 

 life in animals and plants.' (Phys. d. Zeug. p. 160.) Hensen 

 takes as his starting-point the fact that, among many animals, 

 e. g. bees and wasps, parthenogenetic ova give rise to male 

 individuals only, while in others, namely Psyche and Solenobia 

 among Lepidoptera, and Apits, Artejiiia, and Limnadia among 

 Crustacea, only females are thus produced ; further, that in many 

 Lepidoptera, as Liparis, single eggs possess a power of develop- 

 ing without fertilization, but only into male insects, or into 

 caterpillars which afterwards die, or in other cases only as far 

 as some earlier or later phase of embryonic life. From this he 

 concludes that we are here ' dealing with a graduated series of 

 phenomena,' ' with a gradation in the powers of development 

 and of reproduction, that is of qualities which can be con- 

 veniently included in the term ' sexual force. ^ Hence at that 

 time V. Hensen considered, if I have rightly understood him, 

 that the ' sexual force,' it is true, ordinarily reaches the egg by 

 fertilization, but that it may, under certain conditions and in 

 varying degrees, be included in the female germ-cells alone. 

 Such ova can then undergo embryonic development without 

 fertilization, and, according to the amount of contained ' sexual 

 force,' can pass through a longer or shorter period of develop- 

 ment ; many reaching only a certain stage of segmentation, 

 others the entire larval stage, while finally some may attain the 

 condition of imago, with mature sexual organs. There are 

 moreover various degrees of 'sexual force ' ; for Hensen con- 

 siders that male offspring are produced by a smaller force 

 than females. Eggs from which, without fertilization, males 

 only can arise (bees), possess, in his opinion, a smaller 

 'sexual force' than those which without fertilization produce 

 females. This view ultimately depends on the conception 

 of the life-preserving effect of fertilization, since males alone 

 cannot perpetuate the species ; and hence eggs which, without 

 fertilization, give rise to males, are unable to maintain the con- 

 tinuity of life, and would finally result in the disappearance of 

 the species, just as eggs of still smaller ' sexual force ' lead to 

 the disappearance of the individual in the larval or even earlier 

 embryonic stages. 



