September 4, 1914] 



SCIENCE 



321 



have any kind of children within the racial 

 limits is contrary to all experience, yet we 

 have greatly entertained such ideas. As 

 I have said elsewhere, the truth might have 

 been found out at any period in the world's 

 history if only pedigrees had been drawn 

 the right way up. If, instead of exhibiting 

 the successive pairs of progenitors who have 

 contributed to the making of an ultimate 

 individual, some one had had the idea of 

 setting out the posterity of a single ancestor 

 who possessed a marked feature such as the 

 Hapsburg lip, and showing the transmis- 

 sion of this feature along some of t;^e 

 descending branches and the permanent 

 loss of the feature in collaterals, the essen- 

 tial truth that heredity can be expressed in 

 terms of presence and absence must have 

 at once become apparent. For the descend- 

 ant is not, as he appears in the conven- 

 tional pedigree, a sort of pool into which 

 each tributary ancestral stream has poured 

 something, but rather a conglomerate of 

 ingredient-characters taken from his pro- 

 genitors in such a way that some ingredi- 

 ents are represented and others are omitted. 



Let me not, however, give the impression 

 that the unraveling of such descents is 

 easy. Even with fairly fuU details, which 

 in the case of man are very rarely to be 

 had, many complications occur, often pre- 

 venting us from obtaining more than a 

 rough general indication 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 between 

 factors by which the presence of some is 

 masked. 



Necessarily the clearest evidence of regu- 

 larity in the inheritance of human char- 

 acteristics 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 stands almost alone in this respect. 

 With regard to the inheritance of other 

 color-characteristics some advance has been 

 made, but everything points to the infer- 

 ence that the genetics of color and many 

 other features in man will prove excep- 

 tionally complex. There are, however, 

 plenty of indications of system comparable 

 with those which we trace in various ani- 

 mals 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 excep- 

 tions. 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- 

 Bhle, in the course of experiments with cer- 

 tain wheats, that several factors having the 

 same power may co-exist in the same indi- 

 vidual. These cumulative factors do not 

 necessarily produce a cumulative effect, 

 for any one of them may suffice to give the 

 full result. Jiist as the pure-bred tall pea 

 with its two factors for tallness is no taUer 

 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 observa- 

 tions have been made by East and others. 

 In some cases, as in the primulas studied by 

 Gregory, the effect is cumulative. These 



