200 HEREDITY AND ENVIRONMENT 



normal niomber of joints; acondroplasy, or 

 short and crooked limbs, such as occur in cer- 

 tain breeds of dogs and sheep and in certain 

 human dwarfs ; myopia, in which the eyeball is 

 elongated; glaucoma, or swelling of the eye- 

 ball; coloboma, or open suture of the iris; 

 otosclerosis, or thickened tympanic mem- 

 brane, 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 monstrous conditions 

 are due to abnormal environment 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 im^der 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 



