HEEEDITY BATESOK, 379 



Let me not, however, give the impression that the unravelling of 

 such descents is easy. Even with fairly full details, which in the 

 case of man are very rarely to be had, many complications occur, 

 often preventing us from obtaining more than a rough general indica- 

 tion of the system of descent. The nature of these complications we 

 partly understand from our experience of animals and plants which 

 are amenable to breeding under careful restrictions, and we know 

 that they are mostly referable to various effects of interaction be- 

 tween factors by which the presence of some is masked. 



Necessarily the clearest evidence of regularity in the inlieritance 

 of human characteristics has been obtained in regard to the descent 

 of marked abnormalities of structure and congenital diseases. Of 

 the descent of ordinary distinctions such as are met with in the 

 normal healthy population we know little for certain. Hurst's evi- 

 dence, that two parents both with light-colored eyes — in the strict 

 sense, meaning that no pigment is present on the front of the iris — 

 do not have dark-eyed children, still stand almost alone in this re- 

 spect. With regard to the inheritance of other color-character- 

 istics some advance has been made, but everything points to the in- 

 ference that the genetics of color and many other features in man 

 will prove exceptionally complex. There are, however, plenty of 

 indications of system comparable with those which we trace in 

 various animals and plants, and we are assured that to extend and 

 clarify such evidence is only a matter of careful analysis. For the 

 present, in asserting almost any general rules for human descent, 

 we do right to make large reservations for possible exceptions. It 

 is tantalizing to have to wait, but of the ultimate result there can be 

 no doubt. 



I spoke of complications. Two of these are worth illustrating 

 here, for probably both of them play a great part in human genetics. 

 It was discovered by Nilsson-Ehle, in the course of experiments with 

 certain wheats, that several factors having the same power may co- 

 exist in the same individual. These cumulative factors do not neces- 

 sarily produce a cumulative effect, for any one of them may suffice to 

 give the full result. Just as the pure-bred tall pea with its two 

 factors for tallness is no taller than the cross-bred with a single 

 factor, so these wheats with three pairs of factors for red color are 

 no redder than the ordinary reds of the same family. Similar 

 observations have been made by East and others. In some cases, as 

 in the Primulas studied by Gregory, the effect is cumulative. These 

 results have been used with plausibility by Davenport and the 

 American workers to elucidate the curious case of the mulatto. If 

 the descent of color in the cross between the negro and the white man 

 followed the simplest rule, the offspring of two first-cross mulattos 

 would be, on an average, one black, two mulattos, one white, but this 



