xv] Variation 279 



In reply to the question so often asked, Has modern 

 investigation given evidence as to the nature of these 

 causes? the answer must still be, Almost nothing. Signs 

 there are however that the search for such evidence is 

 beginning at last to be not altogether unfruitful, but the 

 detailed consideration of this part of the subject must be 

 postponed to another occasion. 



Reversion occurs when the sum total of the factors 

 returns to that which it has been in some original type. 

 Such a return may be brought about by the omission of an 

 element or elements, as when the rose-comb fowl for any 

 reason has a single-combed offspring. Conversely, the 

 return may occur by the addition of some missing element 

 needed to complete the original type. As yet no means are 

 known by which the omission or addition of elements can 

 be made at will, except by crossing. Reversion on crossing 

 is thus the particular case in which one or more missing 

 factors are brought in by the parents of the cross-bred. 

 The most striking cases of such reversion on crossing are 

 those in which neither parent seems to the observer to 

 contain anything specially reminiscent of the original type, 

 and yet the offspring of the cross are all of that type. Such 

 cases are those of the two white Sweet Peas which, though 

 each severally breeding quite true to whiteness, when 

 crossed together have a reversionary offspring ; or of the 

 two breeds of Pigeon, which though neither has the blue- 

 barred plumage of the Rock Pigeon, yet contain materials 

 from which blue-barred birds may be compounded. 



Not often can we hope to be able to specify the comple- 

 mentary elements which must meet each other in order that 

 a certain compound character may be produced. Neverthe- 

 less, by the co-operation of physiological chemistry with 

 genetics there is every hope that in favourable cases of a 

 simple order actual demonstrations of these elements may 

 be carried out. Perhaps the nearest approach to such an 

 achievement is that made by Miss Wheldale in her experi- 

 ments on Antirrhinum (Snapdragon). Crossing the very 

 pale yellow, known as " Ivory," with a white variety, she 

 obtained the F l generation all of the dull red of common 

 Snapdragons (303)*. The work of Overton and others had 

 * f 2 was of the normal kind expected in such cases. 



