CLASSIFICATION OF VARIATIONS 91 



working of the mechanical process of division, and a factor 

 gets left out, the loss being attested by the appearance of a 

 recessive variety in some subsequent generation. 



Consistently with this presentation of the facts we find that, 

 as in our domesticated animals and plants, a diversity of recessives 

 may appear within a moderately short period, and that when 

 variations come they often do not come alone. Witness the 

 cultural history of the Sweet Pea, Primula Sinensis, Primula 

 obconica, Nemesia strumosa and many such examples in which 

 variation when it did come was abundant. The fact cannot 

 be too often emphasized that in the vast proportion of these 

 examples of substantive variation under domestication, as well 

 as of substantive variation in the natural state, the change 

 has come about by omission, not by addition. To take, for 

 example, the case of the Potato, in which so many spontaneous 

 bud-variations have been recorded, East after a careful study 

 of the evidence has lately declared his belief that all are of this 

 nature, and the opinion might be extended to many other groups 

 of cases whether of bud or seminal variation. Morgan draws 

 the same conclusion in reference to the many varieties he has 

 studied in Drosophila. 



In the Sweet Pea, a form which is beyond suspicion of having 

 been crossed with anything else, and has certainly produced 

 all the multitude of types which we now possess by variations 

 from one wild species, there is only one character of the modern 

 types which could, with any plausibility, be referred to a factor 

 not originally forming part of the constituents of the wild species. 

 This is the waved edge, so characteristic of the "Spencer'* 

 varieties; for the cross between a smooth-edged and a waved 

 type gives an intermediate not unfrequently. Nevertheless 

 there is practically no doubt that this is merely an imperfection 

 in the dominance of the smooth edge, and we may feel sure 

 that any plant homozygous for smooth edge would show no wave 

 at all. Hence it is quite possible that even the appearance of 

 the original waved type, Countess Spencer, was due to the loss 

 of one of the factors for smooth edge at some time in the history 

 of the Sweet Pea. 



