Phenomena of Inheritance 69 



(c) Pathological Peculiarities are really only unusual or abnor- 

 mal anatomical or physiological characters, but they are of such 

 interest and importance as to deserve special mention. Many such 

 abnormalities are undoubtedly inherited, among which are the 

 following : polydactylism, in which more than the normal number 

 of digits are present (Fig. 38) ; syndactylism, or a condition 

 of webbed fingers and toes ; brachydactylism, in which fingers are 

 short and stumpy and usually contain less than the normal num- 

 ber of joints (Fig. 39) ; achondroplasy, or short and crooked 

 limbs, such as occur in certain breeds of dogs and sheep and in 

 certain human dwarfs ; myopia, in which the eyeball is elongated ; 

 glaucoma, or pressure within the eyeball ; coloboma, or open su- 

 ture 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 monstrous conditions are due to abnormal environment and are 

 not inherited. 



Are Diseases Inherited. The question of the inheritance of 

 diseases may be briefly considered here. If a disease is due to 

 some defect in the hereditary constitution, it is inherited ; other- 

 wise, according to our definition of heredity, it is not. Of course 

 no disease develops without extrinsic causes but when one indi- 

 vidual 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 certain diseases run in families and have the ap- 

 pearance of being inherited, but in this case as in many others it 

 is extremely difficult in the absence of experiments to distinguish 

 between effects due to intrinsic causes and those due to extrinsic 

 ones. Where the specific cause of disease is some micro-organism 

 the individual must have been infected at some time or other, al- 

 most invariably after birth. In few instances is the oosperm itself 

 infected, and even when it is, this is not, strictly speaking, a case 



