324 



THE GERM-PLASM 



peared in the grandchild, and had become retransformed into 

 the corresponding characters of the wild species.. Were this 

 so, the reversion might be considered complete, and the same 

 theoretical explanation could be given for it as in the case of 

 the Datura hybrid. Unfortunately Darwin leaves this point 

 untouched, as he devoted his attention chiefly to the coloration 

 so characteristic of the species. It seems to me to be very 

 probable, however, from several of his statements, that this was 

 also essentially a mere case of reversion as regards the colora- 

 tion of the plumage. I conclude this principally from the fact 

 that the blue or original coloration of Columba livia occurs in 

 all the principal breeds of pigeons, although these blue sub- 

 varieties are rare in some of them. The other racial charac- 

 teristics do not at any rate exclude the possibility of a blue 

 coloration : and thus, on the other hand, reversion to the blue 

 colour is not necessarily accompanied by a reversion to all the 

 other characters of the ancestral form. 



It is perfectly certain that in most cases the reversion pro- 

 duced by cross-breeding is not complete, even as regards the 

 coloration, but gradually becomes more marked, so that at 

 first very faint and hardly perceptible indications of the wing- 

 and tail-bars are seen, and these become more pronounced by 

 degrees, so that a partial blue coloration with perfect bars, and 

 finally the perfect slate-colour and complete bars of the ances- 

 tral form are produced. The greater number of reversions in 

 pigeons must consequently be incomplete, — i.e., they must refer 

 to individual characters or groups of characters only, and we 

 are here concerned with the theoretical explanation of such cases 

 as these. 



I take for granted the facts that all valuable races of pigeons 

 breed true, that all the main breeds are characterised by differ- 

 ences in form, and that sub-breeds differ merely as regards 

 colour. In my opinion this implies, in the first place, that the 

 germ-plasm of the main breeds has become essentially modified 

 from that of the rock-pigeon, and that only lesser portions of it 

 correspond to that of the ancestral form ; and also, that all the 

 deter/fiitiants have not become uiodified to an equal extent, — those 

 for the coloration having undergone the least, and those for 

 the whole body the most, alteration. I therefore suppose that 

 the germ-plasm of one of tlie main breeds consists of a number 

 of modified idants, none of which any longer correspond exactly 



