26 PRESIDENT’S ADDRESS. 
theless calls for mention here, because it is an admirable illustration of 
factorial predestination. It moreover exemplifies that parental polarity 
of the zygote to which I alluded in my first Address, a phenomenon 
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 malformations, however, 
we have abundant data for deciding that many are transmitted as 
dominants and a few as recessives. The most remarkable collection 
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 
collected 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 formule 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, either 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 
individual * 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. 
* The first human descent proved to follow Mendelian rules was that of a 
serious malformation of the hand studied by Farabee in America. Drinkwater 
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. 
