90 ON HEREDITY, [II. 



variations of cliaracters are transmitted without natural se- 

 lection playing any active part in the change. Such a case 

 is afforded by the short-sightedness so common in civilized 

 nations. 



This affection is certainly hereditary in some cases, and 

 it may well have been explained as an example of the trans- 

 mission of acquired changes. It has been argued that ac- 

 quired short-sightedness can be in a slight degree transmitted, 

 and that each successive generation has developed a further 

 degree of the disease by habitually holding books etc. close 

 to the eyes, so that the inborn predisposition to short-sighted- 

 ness is continually accumulating. 



But we must remember that variations in the refraction of 

 the human eye have been for a long time independent of the 

 preserving control of natural selection. In the struggle for 

 existence, a blind man would certainly disappear before those 

 endow^ed with sight, but myopia does not prevent any one 

 from gaining a living. 



A short-sighted lynx, hawk, or gazelle, or even a short- 

 sighted Indian, would be eliminated b}^ natural selection, but 

 a short-sighted European of the higher class finds no difficulty 

 in earning his bread. 



Those fluctuations on either side of the average which 

 we call myopia and hypermetropia occur in the same manner, 

 and are due to the same causes, as those which operate in 

 producing degeneration in the eyes of cave-dwelling animals. 

 If, therefore, we not infrequently meet with families in which 

 myopia is hereditary, such results may be attributed to the 

 transmission of an accidental disposition on the part of the 

 germ, instead of to the transmission of acquired short- 

 sightedness. A very large proportion of short-sighted people 

 do not owe their affliction to inheritance at all, but have ac- 

 quired it for themselves ; for there is no doubt that a normal 

 eye may be rendered mj^opic in the course of a life-time by 

 continually looking at objects from a very short distance, even 

 when no hereditary predisposition towards the disease can 

 be shown to exist. Such a change would of course appear 

 more readily if there was also a corresponding predisposition 

 on the part of the eye. But I should not explain this widely 

 spread predisposition towards myopia as due to the trans- 



