2o6 J. ARTHUR THOMSON [march 



It must be carefully noted that the experiments have been numerous, 

 for it is only thus that the element of coincidence can be duly allowed 

 for. And the general result stands out clearly that the reversion- 

 interpretation has received substantial support. 



At the same time it seems extremely doubtful whether such 

 a phenomenon as a complete ulna in a horse, and other cases of the 

 same sort which are cited, furnish any relevant evidence of reversion. 



Prepotency. 



When an organism of either sex shows great power in transmitting 

 its individual characteristics, it is said to be prepotent as regards these. 

 A stallion or a mare, a bull or a cow, may be so prepotent that its 

 characters reappear in a high percentage of the offspring, no matter 

 what the other parent may be. 



It seems doubtful whether anything beyond convenience is gained 

 by the word prepotency — since all these general terms are apt to form 

 the dust-particles of intellectual fog ; what we have to do with is the 

 fact that certain variations are markedly stable, heritable, and per- 

 sistent, almost aggressively persistent one might say. It seems likely 

 that they express positions of relatively great organic equilibrium. 



The quality of prepotency is obviously a relative one, and only 

 verifiable in its results. That is to say, it is never more than probable 

 in its exercise. Nor are we able, at the present stage of biological 

 analysis, to define with any precision wherein the secret of prepotency 

 actually lies. We have hardly got beyond imagining that there is a 

 struggle in the germ-cells before and after fertilisation, and that there 

 is a survival of the fitter components within the microcosm of the ovum 

 just as in the macrocosm outside. 



Yet, in spite of the obscurity which shrouds the interpretation, the 

 fact of prepotency is certain, and it is of direct human interest not only 

 in connection with the breeding of stock, but also as regards the 

 evolution of races of men. The stud-books show the enormous value 

 of a prepotent sire, and we may regard the persistence of a Celtic, 

 Semitic, or Gipsy strain, in spite of complex intercrossing in the 

 pedigree, as an illustration of prepotency. 



This quality, so potent for good or ill, may arise in one of two 

 ways — (a) as an attribute of a " sport " or discontinuous variant, or 

 (b) as the result of inbreeding. As to the former, Standfuss, who has 

 had so much experience in hybridising butterflies, says that when a 

 sporadic variety is crossed with the normal form of the stock the result 

 is that the offspring agree either with the normal form or with the 

 sport, intermediate forms being absent, [" Handbuch der palaeark- 

 tischen Gross-Schmetterlinge," 2nd edition. Jena, 1896]. Similarly, 

 Mr. J. W. Tutt reports among the conclusions of Dr. Biding and Mr. 

 Bacot as to hybridising species of Tcplirosia, that while the (phylo- 



