24 % PRESIDENT’S ADDRESS. 
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 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 nature of the case strict proof is not to be had. 
But at present I own to a preference for regarding such examples as 
