ON HEREDITY. 89 



which, according to Marsh 1 , consists only of a very thin and 

 relatively small humerus, which was probably concealed beneath the 

 skin. The water-fleas (Dapknidae) possess in the embryonic state 

 three complete and almost equal pairs of jaws, but two of these 

 entirely disappear, and do not develope into jaws in any species. 

 In the same way, the embryo of the maggot-like legless larva of 

 bees and wasps possesses three pairs of ancestral limbs. 



There are, however, cases in which, apparently, acquired variations 

 of characters are transmitted without natural selection 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 transmission of 

 acquired changes. It has been argued, that acquired short-sighted- 

 ness can be in a slight degree transmitted, and that each successive 

 generation has developed a further degree of the disease by habitu- 

 ally holding books etc. close to the eyes, so that the inborn pre- 

 disposition to short-sightedness 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 pre- 

 serving control of natural selection. In the struggle for existence, 

 a blind man would certainly disappear before those endowed 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 by 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 nx y 

 be attributed to the transmission of an accidental disposition on tfcti 

 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 acquired it 

 for themselves ; for there is no doubt that a normal eye may be 



1 0. C. Marsh, ' Odontornithes, a Monograph on the extinct toothed Birds of 

 North America,' Washington, 1880. 



