BBITISH ASSOCIATION. 349 



tributary ancestral stream has poured something, but rather a con- 

 glomerate of ingredient-characters taken from his progenitors in such 

 a way that some ingredients are represented and others are omitted. 



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 

 indication of the system of descent. The nature of these compli- 

 cations 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 regularity in the inheritance 

 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 evidence, that 

 two parents both with light-coloured 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 colour- characteristics some 

 advance has been made, but everything points to the inference that 

 the genetics of colour 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 tantalising 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 

 necessarily 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 colour 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 colour 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 is notoriously not so. Evidence of some segregation 

 is fairly clear, and the deficiency of real whites may perhaps be 

 accounted for on the hypothesis of cumulative factors, though by the 



