676 
whatever their own parentage, do not have colour- 
blind descendants, unless they marry transmitting 
women. There are points still doubtful in the inter- 
pretation, but the critical fact is clear, that the germ- 
cells of the colour-blind man are of two kinds: 
(i) those which do not carry on the affection and are 
destined to take part in the formation of sons; and 
(ii) those which do carry on the colour-blindness and 
are destined to form daughters. There is evidence 
that the ova also are similarly predestined to form 
one or other of the sexes, but to discuss the whole 
question of sex-determination is beyond my present 
scope. The descent of these sex-limited affections 
nevertheless calls for mention here, because it is an 
admirable illustration of factorial predestination. 
moreover exemplifies that parental polarity of the 
zygote to which I alluded in my first Address, a phe- 
nomenon which we suspect to be at the bottom of 
various anomalies of heredity, and suggests that there 
may be truth in the popular notion that in some 
respects sons resemble their mothers and daughters 
their fathers. 
As to the descent of hereditary diseases and mal- 
formations, however, we have abundant data for de- 
ciding that many are transmitted as dominants and 
a few as recessives. The most remarkable coliection 
of these data is to be found in family histories of 
diseases of the eye. Neurology and dermatology have 
also contributed many very instructive pedigrees. In 
great measure the ophthalmological material was col- 
lected by Edward Nettleship, for whose death we so 
lately grieved. After retiring from practice as an 
oculist he devoted several years to this most laborious 
task. He was not content with hearsay evidence, 
but travelled incessantly, personally examining all 
accessible members of the families concerned, working 
in such a way that his pedigrees are models of orderly 
observation and recording. His zeal stimulated many 
younger men to take part in the work, and it will now 
go on, with the result that the systems of descent 
of all the common hereditary diseases of the eye will 
soon be known with approximate accuracy. 
Give a little imagination to considering the chief 
deduction from this work. Technical details apart, 
and granting that we cannot wholly interpret the 
numerical results, sometimes noticeably more and 
sometimes fewer descendants of these patients being 
affected than Mendelian formulz would indicate, the 
expectation is that in the case of many diseases of 
the eye a large proportion of the children, grand- 
children, and remoter descendants of the patients will 
be affected with the disease. Sometimes it is only 
defective sight that is transmitted; in other cases it is 
blindness, eiher from birth or coming on at some later 
age. The most striking example perhaps is that of 
a form of night-blindness still prevalent in a district 
near Montpellier, which has affected at least 130 
persons, all descending from a single affected indi- 
vidual! who came into the country in the seventeenth 
century. The transmission is in every case through 
an affected parent, and no normal has been known 
to pass on the condition. Such an example well serves 
to illustrate the fixity of the rules of descent. Similar 
instances might be recited relating to a great variety 
of other conditions, some trivial, others grave. 
At various times it has been declared that men are 
born equal, and that the inequality is brought about 
by unequal opportunities. | Acquaintance with the 
pedigrees of disease soon shows the fatuity of such 
1 The first human descent proved to follow Mendelian rules was that of a 
serious mal‘ormation of the hand studied by Farabee in America. Drink- 
water subsequently worked out pedigrees for the same malformation in 
England. After many attempts, he now tells me that he has succeeded in 
proving that the American family and one of his own had an abnormal 
ancestor in common, five generations ago. 
NO.) 2330) VOL. 492] 
NATOK 
It | 
without restraint. 
[AUGUST 27, I914 
fancies. The same conclusion, we may be sure, 
would result from the true representation of the de- 
scent of any human faculty. Never since Galton’s 
publications can the matter have been in any doubt. 
At the time he began to study family histories even 
the broad significance of heredity was frequently 
denied, and resemblances to parents or ancestors were 
looked on as interesting curiosities. Inveighing 
against hereditary political institutions, Tom Paine 
remarks that the idea is as absurd as that of an 
‘hereditary wise man,” or an “hereditary mathema- 
tician,’”’ and to this day I suppose many people are 
not aware that he is saying anything more than 
commonly foolish. We, on the contrary, would feel 
it something of a puzzle if two parents, both mathe- 
matically gifted, had any children not mathematicians. 
Galton first demonstrated the overwhelming import- 
ance of these considerations, and had he not been 
misled, partly by the theory of pangenesis, but more 
by his mathematical instincts and training, which 
prompted him to apply statistical treatment rather 
than qualitative analysis, he might, not improbably, 
have discovered the essential facts of Mendelism. 
It happens rarely that science has anything to offer 
to the common stock of ideas at once so comprehen- 
sive and so simple that the courses of our thoughts 
are changed. Contributions to the material progress 
of mankind are comparatively frequent. They result 
at once in application. Transit is quickened; com- 
munication is made easier; the food-supply is in- 
creased and population multiplied. By direct appli- 
cation to the breeding of animals and plants such 
results must even flow from Mendel’s work. But I 
imagine the greatest practical change likely to ensue 
from modern genetic discovery will be a quickening of 
interest in the true nature of man and in the biology 
of races. I have spoken cautiously as to the evidence 
for the operation of any simple Mendelian system in 
the descent of human faculty; yet the certainty that 
systems which differ from the simpler schemes only 
in degree of complexity are at work in the distribution 
of characters among the human population cannot fail 
co influence our conceptions of life and of ethics, lead- 
ing perhaps ultimately to modification of social usage. 
That change cannot but be in the main one of simplifi- 
cation. The eighteenth century made great pretence 
of a return to nature, but it did not occur to those 
philosophers first to inquire what nature is; and per- 
haps not even the patristic writings contain fantasies 
much further from physiological truth than those 
which the rationalists of the ‘t‘ Encyclopedia ’’ adopted 
as the basis of their social schemes. For men are so 
far from being born equal or similar that to the 
naturalist they stand as the very type of a polymorphic 
species. Even most of our local races consist of many 
distinct strains and individual types. From the popu- 
lation of any ordinary English town as many distinct 
human breeds could in a few generations be isolated 
as there are now breeds of dogs, and indeed such a 
population in its present state is much what the dogs 
of Europe would be in ten years’ time but for the inter- 
ference of the fanciers. Even as at present consti- 
tuted, owing to the isolating effects of instinct, 
fashion, occupation, and social class, many incipient 
strains already exist. 
In one respect civilised man differs from all other 
species of animal or plant in that, having prodigious 
and ever-increasing power over nature, he invokes 
these powers for the preservation and maintenance of 
many of the inferior and all the defective members of 
his species. The inferior freely multiply, and the 
defective, if their defects be not so grave as to lead 
to their detention in prisons or asylums, multiply also 
Heredity being strict in its action, 
