HEEEDITY BATESON. 381 



blindness and are destined to form daughters. There is evidence 

 that the OA^a 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 illus- 

 tration of factorial predestination. It moreover exemplifies that 

 parental polarity of the zygote, to which I alluded in my first ad- 

 dress—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, how- 

 ever, we have abundant data for deciding that many are transmitted 

 as dominants and a few as recessives. The most remarkable collec- 

 tion of these data is to be found in family histories of diseases of the 

 eye. Neurolog}^ and dermatology have also contributed many very 

 instructive pedigrees. In great measure the ophthalmological ma- 

 terial was collected by Edw^ard 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 traveled 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 can not 

 wholly interpret the numerical results, sometimes noticeably more 

 and sometimes fewer descendants of these patients being affected 

 than Mendelian formulae would indicate, the expectation is that in 

 the case of many diseases of the eye a large proportion of the chil- 

 dren, grandchildren, 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 com- 

 ing on at some later age. The most striking example perhaps is that 

 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 



1 The first human descont proved 1o follow Mendelian rules was that of a serious mal- 

 formation of the hand studied by Farabee in America. Driukwater 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. 



