306 HEREDITY AND ENVIRONMENT 



normal number of joints (Fig. 67) ; acondro- 

 plasy, or short and crooked limbs, such as occur 

 in certain breeds of dogs and sheep and in cer- 

 tain human dwarfs (Fig. 68) ; myopia, in 

 which the eyeball is elongated; glaucoma, or 

 pressure within the eyeball; coloboma, or open 

 suture of the iris; otosclerosis, or rigidity of 

 tympanum and ossicles, causing "hardness of 

 hearing"; some forms of deaf -mutism, due to 

 certain defects of the inner ear; and many 

 other characters too numerous to mention here. 

 On the other hand many abnormal or mon- 

 strous conditions are due to abnormal environ- 

 ment and are not inherited. 



The question of the inheritance of diseases 

 may be briefly considered here. If a disease is 

 due to some defect in the hereditary constitu- 

 tion, it is inherited; otherwise, according to 

 our definition of heredity, it is not. Of course 

 no disease develops without extrinsic causes 

 but when one individual takes a disease while 

 another under the same conditions does not, 

 the differential cause may be an inherited one, 

 or it may be due to differences in the previous 

 conditions of life. There is no doubt that 



