THE PHENOMENA OF REVERSION 323 



five of the latter belong to D. Icevis and five to D. ferox, and 

 these are consequently dissimilar, the six similar ancestral idants, 

 which have a cumulative effect, will preponderate, because the 

 2x5 different idants of D. Icevis and D.fet'ox do not produce 

 the cumulative effect of 10. 



Hitherto I have assumed that we are in this case dealing 

 with a complete reversion to the ancestral form, and not merely 

 with reversion to individual ancestral characters. I cannot, 

 however, judge with certainty in this respect from the facts 

 known to me concerning these hybrids ; and as this instance 

 was not chosen for its own sake, but merely as an example in 

 which complete reversion to more remote ancestors might be 

 accounted for, it must remain undecided whether it really belongs 

 to the above category, and whether such instances of complete 

 reversion actually occur. The blue colour of the flowers is at 

 any rate not the only apparently new character in these hybrids ; 

 the stem, for instance, is brown, while it is green in the two 

 pure species. In only one of these species {D. ferox) is the stem 

 brown at first, and this colour is only retained as a brown ring 

 around the base of the stem. I must leave botanists to decide 

 whether the shape of the leaves, and the structure of the stem or 

 fruit, afford any reason for considering these parts as intermediate 

 between those of the two parental species, or whether they are 

 to be regarded as deviations from both, and presumably, there- 

 fore, as reversions to the ancestral species. 



It may in all probability be assumed that the process is not 

 a complete one in the above-mentioned cases in which reversion 

 to the wild ancestral form in different races often occurs when 

 the latter are crossed. Darwin certainly gives one instance of a 

 pigeon which ' was hardly distinguishable from the wild Shetland 

 species,' but which was, nevertheless, descended from four 

 grandparents which differed very considerably from the wild 

 species {Colieniba livia)* This bird, which was blue, and pos- 

 sessed the typical black bars on the wings and tail, was descended 

 from a red spot, a white fantail, and two black barbs. These 

 breeds, as is well known, differ from the wild pigeon in colour 

 as well as in many other details, such as the length of the beak 

 and number of tail feathers ; and it would therefore be interest- 

 ing to ascertain whether these racial characters had all disap- 



* 'Animals and Plants under Domestication,' p. 14. 



