306 THE GERM-PLASM 



hybrid into one of the ancestral forms." As these investigators 

 were only concerned with obtaining a complete return to the 

 ancestral form, they always selected for purposes of propagation 

 those plants which bore the closest resemblance to the desired 

 parental type. They, nevertheless, still obtained individual 

 plants which bore traces of the other ancestral species, up to 

 the fifth generation, just as would be expected on theoretical 

 grounds. 



The agreement with our theorv extends even further than 

 this : — i)idi7>idual cases of complete reversion to the ancestral 

 form must theoretically occur ifi the second generation^ a?id such 

 cases have actually been observed. Godron, indeed, found that 

 ' Melandrynm album x rubru?n., even when fertilised by its 

 own pollen, reverted in the second generation to the ancestral 

 species.' * This is easily explained on the basis of our theory. 

 If the reducing division took place in certain of the mother- 

 oferm-cells of the hvbrid in such a manner that each of the two 

 mature germ-cells contained \ idants f of J/, album or M. rubrum 

 only, two possible combinations might occur in the fertilisation 

 of these germ-cells : — either a germ-cell with idants of J/. 

 rubrum would meet with another containing idants of M. album, 

 or else both the conjugating cells would contain idants of the 

 same species. In the former case, the hybrid form would again 

 be produced, while in the latter, the pure ancestral form would 

 result. Such reversions seem to be of rare occurrence, and this 

 may indicate that when they do occur, some unknown conditions 

 favour the halving of the germ-plasm in the mother-germ-cells 

 along a defi)iite plane of section, viz., that which passes between 

 the groups of paternal and maternal idants. 



Although not proved by actual observation, it is apparent 

 from our theory that the hybrid may revert co7)ipletely to the 

 ancestral form. This would be impossible if the reducing 

 division did not occur, or if the germ-plasm consisted of a 

 homogeneous mass which became completely mixed with that 

 of the other species in the process of hybridisation. The germ- 

 plasm of the one species would then, moreover, never be removed 

 by means of the reducing division : — the reduction would be 

 quantitative, and not a qualitative one. Reversion to the an- 

 cestral form could then in no case occur on fertilisation taking 



* Focke, p. 485. 



t n signifies the normal number of idants in the species in question. 



