AUGUST 27, 1914] 
conceptions of the nature of man by which they were 
discouraged from the application of any rigorous 
analysis. Medical literature still abounds with traces 
of these archaisms, and, indeed, it is only quite 
recently that prominent horse-breeders have come to 
see that the dam matters as much as the sire. For 
them, though vast pecuniary considerations were in- 
volved, the old ‘homunculus’? theory was good 
enough. We were amazed at the notions of genetic 
physiology which Prof. Baldwin Spencer encountered 
in his wonderful researches among the natives of 
Central Australia; but in truth, if we reflect that 
these problems have engaged the attention of civilised 
man for ages, the fact that he, with all his powers 
of recording and deduction, failed to discover any 
part of the Mendelian system is almost as amazing. 
The popular notion that any parents can have any 
kind of children within the racial limits is contrary 
to all experience, yet we have gravely entertained such 
ideas. As I have said elsewhere, the truth might 
have been found out at any period in the world’s his- 
tory 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 Habsburg lip, 
and showing the transmission of this feature along 
some of the descending branches and the permanent 
loss of the feature in collaterals, the essential truth 
that heredity can be expressed in terms of presence 
and absence must have at once become apparent. For 
the descendant 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 progenitors in such a way that some in- 
gredients 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 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 abnorm- 
alities 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 gene- 
tics 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 evi- 
dence 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 reserva- 
tions for possible exceptions. It is tantalising to have 
to wait, but of the ultimate result there can be no 
doubt. 
Zz 
E2339, VOL ?93i| 
NATORE 
again transmitters. 
675 
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 segrega- 
tion 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 in- 
stances of imperfect segregation. The series of germ- 
cells produced by the cross-bred consists of some with 
no black, some with full black, and others with in- 
termediate quantities of black. No statistical tests 
of the condition of the gametes in such cases exist, 
and it is likely that by choosing suitable crosses 
all sorts of conditions may be found, ranging from 
the simplest case of total segregation, in which there 
are only two forms of gametes, up to those in which 
there are all intermediates in various proportions. 
This at least is what general experience of. hybrid 
products leads me to anticipate. Segregation is some- 
how effected by the rhythms of cell-division, if such 
an expression may be permitted. In some cases the 
whole factor is so easily separated that it is swept 
out at once; in others it is so intermixed that gametes 
of all degrees of purity may result. That is admit- 
tedly a crude metaphor, but as yet we cannot sub- 
stitute a better. Be all this as it may, there are many 
signs that in human heredity phenomena of this kind 
are common, whether they indicate a multiplicity of 
cumulative factors or imperfections in segregation. 
Such phenomena, however, in no way detract from 
the essential truths that segregation occurs, and that 
the organism cannot pass on a factor which it has 
not itself received. 
In human heredity we have found some examples, 
and I believe that we shall find many more, in which 
the descent of factors is limited by sex. The classical 
instances are those of colour-blindness and hzmo- 
philia. Both these conditions occur with much 
greater frequency in males than in females. Of 
colour-blindness at least we know that the sons of the 
colour-blind man do not inherit it (unless the mother 
is a transmitter) and do not transmit it to their 
children of either sex. Some, probably all, of the 
daughters of the colour-blind father inherit the 
character, and though not themselves colour-blind, 
they transmit it to some (probably, on an average, 
half) of their offspring of both sexes. For since these 
normal-sighted women have only received the colour- 
blindness from one side of their parentage, only half 
their offspring, on an average, can inherit it. The 
sons who inherit the colour-blindness will be colour- 
blind, and the inheriting daughters become themselves 
Males with normal colour-vision, 
