THE PHENOMENA OF REVERSION 32 I 



A large number of cases of heredity which have not hitherto 

 been understood can thus be explained in a very simple way. 

 Take, for example, the varying degrees of certainty with luhich 

 varieties of cultivated plants transmit their peculiarities. The 

 extent to which such peculiarities are transmitted must depend 

 on the number of idants in the germ-plasm which have remained 

 unmodified, and the greater this number, the more easily will 

 reversion occur. 



These theoretical considerations will probably account for the 

 first of the three cases which were mentioned above as types of 

 reversion to more remote ancestors, viz., that of Datura ferox 

 X Icevis. The two species of Datura have white flowers, but 

 produce hybrids in which the flowers are blue ; and this is not 

 only occasionally the case, but occurred in every one of the 205 

 plants which Naudin raised from this cross,* and also in all 

 those cases which had been observed previously by Kolreutter 

 and Gartner. t If we assume that, in addition to their own 

 specific idants. the two species of Datura contain a certain 

 number of ancestral idants. the latter might be relatively in- 

 creased in individual germ-cells in consequence of the reducing 

 division : and when these met with germ-cells of the other 

 species, which also contain numerous ancestral idants. a germ- 

 plasm possessing a larger number, and perhaps even a majority, 

 of these idants would be constituted. Individual cases of re- 

 version to the common ancestral form might thus occur. But 

 this assumption is evidently insufficient to account for the facts, 

 for the blue colour of the flower appears in all the hybrids. 

 The reversion in these cases must therefore be independent of 

 a greater accumulation of ancestral idants whicli may possibly 

 occur in individual gemi-cells in consequence of the reducing 

 division. These idants must, on the contrary, come together in 

 each fertilised egg-cell in a sufficient number to preponderate over 

 the modified idants, and to control the ontogeny. They cannot 

 possibly, however, be stronger tiumerically in every case, and 

 another factor must therefore take part in the process, which 

 causes the ancestral primary constituents to preponderate in 

 every case ; and this is in all probability the specific diversity of 

 the modified idants. We have assumed from the first that 



L 



♦Darwin, 'Animals and Plants," Vol. II., p. 254. 

 t Cf. Focke, ' Pflanzenmischlingc," p. 269. 



