314 THE ZOOLOGIST. 



which a single wild origin may with much confidence be assumed. 

 In spite of repeated trials, no one has yet succeeded in crossing the 

 Sweet Pea with any other leguminous species. We know that early 

 in its cultivated history it produced at least two marked varieties 

 which I can only conceive of as spontaneously arising, though, no 

 doubt, the profusion of forms we now have was made by the crossing 

 of those original varieties. I mention the Sweet Pea thus promi- 

 nently for another reason, that it introduces us to another though 

 subsidiary form of variation, which may be described as a fractiona- 

 tion of factors. Some of my Mendelian colleagues have spoken of 

 genetic factors as permanent and indestructible. Eelative permanence 

 in a sense they have, for they commonly come out unchanged after 

 segregation. But I am satisfied that they may occasionally undergo 

 a quantitative disintegration, with the consequence that varieties are 

 produced intermediate between the integral varieties from which 

 they were derived. These disintegrated conditions I have spoken of 

 as subtraction — or reduction — stages. For example, the Picotee 

 Sweet Pea, with its purple edges, can surely be nothing but a 

 condition produced by the factor which ordinarily makes the fully 

 purple flower, quantitatively diminished. The pied animal, such as 

 the Dutch Rabbit, must similarly be regarded as the result of partial 

 defect of the chromogen from which the pigment is formed, or con- 

 ceivably of the factor which effects its oxidation. On such lines I 

 think we may with great confidence interpret all those intergrading 

 forms which breed true and are not produced by factorial interference. 

 It is to be inferred that these fractional degradations are the con- 

 sequence of irregularities in segregation. We constantly see irregu- 

 larities in the ordinary meristic processes, and in the distribution of 

 somatic differentiation. We are familiar with half segments, with 

 imperfect twinning, with leaves partially petaloid, with petals par- 

 tially sepaloid. All these are evidences of departures from the normal 

 regularity in the rhythms of repetition, or in those waves of differen- 

 tiation by which the qualities are sorted out among the parts of the 

 body. Similarly, when in segregation the qualities are sorted out 

 among the germ-cells in certain critical cell-divisions, we cannot 

 expect these differentiating divisions to be exempt from the imper- 

 fections and irregularities which are found in all the grosser divisions 

 that we can observe. If I am right, we shall find evidence of these 

 irregularities in the association of unconformable numbers with the 

 appearance of the novelties which I have called fractional. In 

 passing, let us note how the history of the Sweet Pea belies those 

 ideas of a continuous evolution with which we had formerly to 

 contend. The big varieties came first. The little ones have arisen 

 later, as I suggest by fractionation. Presented with a collection of 

 modern Sweet Peas, how prettily would the devotees of Continuity 

 have arranged them in a graduated series, showing how every inter- 

 grade could be found, passing from the full colour of the wild Sicilian 

 species in one direction to white, in the other to the deep purple of 

 " Black Prince," though happily we know these two to be among the 

 earliest to have appeared. 



