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SCIENCE 



[N. S. Vol. XL. No. 1026 



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 prominently for another 

 reason, that it introduces us to another 

 though subsidiary form of variation, which 

 may be described as a fractionation of 

 factors. Some of my Mendelian colleagues 

 have spoken of genetic factors as perma- 

 nent and indestructible. Relative perma- 

 nence in a sense they have, for they com- 

 monly come out unchanged after segrega- 

 tion. But I am satisfied that they may 

 occasionally undergo a quantitative dis- 

 integration, with the consequence that vari- 

 eties are produced intermediate between 

 the integral varieties from which they were 

 derived. These disintegrated conditions I 

 have spoken of as subtraction — ^or reduc- 

 tion — 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 dimin- 

 ished. 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 oxi- 

 dation. On such lines I think we may with 

 great confidence interpret all those inter- 

 grading forms which breed true and are 

 not produced by factorial interference. 



It is to be inferred that these fractional 

 degradations are the consequence of irreg- 

 ularities in segregation. We constantly 

 see irregularities in the ordinary meristie 

 processes, and in the distribution of somatic 

 differentiation. We are familiar with half 

 segments, with imperfect twinning, with 

 leaves partially petaloid, with petals 



partially sepaloid. All these are evidences 

 of departures from the normal regularity 

 in the rhythms of repetition, or in those 

 waves of differentiation 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 

 can not expect these differentiating divi- 

 sions to be exempt from the imperfections 

 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 appear- 

 ance 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 vari- 

 eties came first. The little ones have arisen 

 later, as I suggest by fractionation. Pre- 

 sented with a collection of modern sweet 

 peas how prettily would the devotees of 

 continuity have arranged them in a gradu- 

 ated series, showing how every intergrade 

 could be found, passing from the full color 

 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. 



Having in view these and other consid- 

 erations which might be developed, I feel 

 no reasonable doubt that though we may 

 have to forego a claim to variations by addi- 

 tion of factors, yet variation both by loss 

 of factors and by fractionation of factors 

 is a genuine phenomenon of contemporary 

 nature. If then we have to dispense, as 

 seems likely, with any addition from with- 

 out we must begin seriously to consider 

 whether the course of evolution can at all 

 reasonably be represented as an unpacking 

 of an original complex which contained 



