32 Mutual Independence of Hereditary Characters 



mens. The first method is applied in improving races, the 

 latter in fixing newly acquired varieties. 



To maintain a species with the required proportion of 

 all its hereditary factors, only an occasional crossing is 

 necessary. It need not precede every generation. Where 

 sexual generations alternate with asexual ones, as in the 

 gall-fly, and even where the latter occur in the majority, 

 as in many aphids, this is clearly seen. 



With bees the fertilized eggs become females, the un- 

 fertilized ones males. But since every male descends 

 necessarily from a female that originated through fer- 

 tilization, it evidently profits sufficiently by the advant- 

 ages of an occasional crossing. The aphids, in which the 

 male as well as the female originate parthenogenetically, 

 teach us that here we have to do not with fundamental 

 relations, but with special adaptations. 



The never-opening, so-called cleistogamous flowers, 

 the numerous devices for insuring self-fertilization in 

 flowers in case they are not visited by insects, and the 

 almost unlimited use of vegetative multiplication in plants, 

 all serve to teach us that an occasional fertilization is all 

 that is necessary for the normal preservation of the spe- 

 cies. That in higher animals every individual originates 

 in the sexual way, is therefore obviously only a special 

 adaptation. 



In summarizing the result of these considerations, we 

 may say that the true essence of fertilization consists in 

 V mixing the hereditary characters of the different individ- 

 uals of a species. Hybrids have taught us how we are to 

 conceive this co-mingling. There is no doubt that the pro- 

 cess of mixing is, in principle, the same in both cases. 

 And just as Wichura 17 succeeded in producing hybrids 



17 Wichura, Max. Bastardbefruchtung im Pflanzenreich er- 

 lautert an den Bastarden der Weiden. Breslau, 1865. 



