SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 55 



If two white-flowered species of thorn-apple, Datura ferox and 

 Datura loivis, be crossed, there arises a hybrid with bluish violet 

 flowers and brown stalks instead of green. This was interpreted 

 by Darwin as a reversion to violet-flowering ancestral species on 

 both sides, for there are even now a great number of species of 

 Datura with violet flowers and brown stalks. When the two 

 white forms are crossed the reversion takes place every time, not 

 merely in some cases, and we may conclude from this that in both 

 these species there is still such a strong admixture of the same 

 unvaried ancestral ids that they always excel the ids of the two 

 modern species crossed, in strength though certainly not in number. 

 And this superiority must again depend on the fact that similar 

 determinants of the same part are cumulative in their effect, while 

 dissimilars are not. 



For this reason reversions to remote ancestors occur readily when 

 species and breeds are crossed, while they are rare in the normal in- 

 breeding of a species. The reversions of the breeds of pigeon to their 

 wild ancestral form, the slaty-blue rock-pigeon, never result, as 

 Darwin showed, and as we have already noticed, from pure breeding of 

 one race, but only when two or more breeds are repeatedly crossed 

 with one another. Even then it occurs by no means always, but only 

 now and again. The germ-plasm of the breeds must therefore still 

 contain ids of the rock-pigeon, but in a small number, varying from 

 individual to individual. If by fortunate reducing divisions and the 

 meeting together of a sperm-cell rich in ancestral ids with a similarly 

 endowed egg-cell the number of ancestral ids be raised to such a point 

 that it exceeds the number of modern-breed ids contained in each one 

 of the conjugating germ-cells, the ancestral ids control the develop- 

 ment and reversion occurs, for the ancestral ids together have a cumu- 

 lative effect while the ids of the two parent breeds are different and 

 therefore, as far as they are so, cannot be co-operative in their 

 influence. But it must be understood that they need not be different 

 as far as all their determinants are concerned, but usually only as 

 regards some groups, and thus it happens that reversion does not 

 occur in regard to all, but only in regard to particular characters — 

 thus in the Datura hybrids, chiefly in regard to the colour of the 

 flowers and the stem, and in the hybrids between different breeds 

 of pigeon mainly in regard to the colour and marking of the 

 plumage. 



The reversions of the horse and ass to striped ancestors, which 

 Darwin has made famous, go much further back into the ancestral 

 history of the species, for while we know the ancestral form of the 



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