40 Heredity. 



This explains why, in Germany, myopia is not a reason for 

 rejection by the examining boards. Since constant study creates 

 myopia, and heredity most frequently perpetuates it, the number 

 of short-sighted persons must necessarily increase in a nation 

 devoted to intellectual pursuits. 



2. Anaesthesia of the nerves of sight is transmissible in all its 

 grades and in all its forms. It is a well-known fact that the 

 sensibility of the eye to light is very different in different persons. 

 It may vary as much as 200 per cent, and, of course, will pass 

 through all the intermediate degrees. Heredity transmits these 

 inequalities, from partial to total anaesthesia, or blindness, when 

 the eye, incapable of noting form or colour, has only an indistinct 

 perception of light. 



Congenital blindness may run in families. Blind persons will 

 sometimes beget blind children. A blind beggar was the father 

 of four sons and a daughter, all blind. 1 Dufau, in his work on 

 Blindness, cites the cases of 2 1 persons blind from birth, or soon 

 after, whose ancestors father, mother, grandparents, and uncles 

 had some serious affection of the eyes. 



Amaurosis, nyctalopia, and cataract in the parents may become 

 blindness in the children ; and such transformations of heredity 

 are not rare in animals. 



The incapacity to distinguish colours, known under the name of 

 Daltonism, or colour-blindness is notoriously hereditary. The 

 distinguished English chemist Dalton was so affected, as were 

 also two of his brothers. Sedgwick discovered that colour-blind- 

 ness occurs oftener in men than in w T omen. In eight families akin 

 to each other, this affection lasted through five generations, and 

 extended to 71 persons. 2 



It is readily understood that such an anomaly of vision is not 

 without influence on the mind, at least from the aesthetic point of 

 view. An old man, who had from childhood observed that he 

 could not call the various colours by their names, was grieved 

 because he saw nothing in paintings but what was gray and 

 sombre in a landscape only an obscure haze, in the sunrise and 



1 Lucas, i. 404. 



8 Darwin, Variation^ etc. t ii. p. 70. 



