PHENOMENA OF INHERITANCE 3*7 



man dwarfs; myopia in which the eyeball is elongated; glaucoma or 

 swelling of the eyeball ; coloboma, or open suture of the iris ; otosclerosis, 

 or thickened tympanic membrane, 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 en- 

 vironment 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 constitution, 

 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 certain diseases run in families and have the appearance 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 intrin- 

 sic causes and those due to extrinsic ones. Where the specific cause of a 

 disease is some microorganism the individual must have been infected 

 at some time or other, almost invariably after birth. In few instances, 

 is the oosperm itself infected, and even when it is this is not, strictly 

 speaking, a case of inheritance, but rather one of early infection. Pear- 

 son has found that there is a marked correlation (represented by the 

 number .55 when complete correlation is 1) between the tuberculous 

 parents and tuberculous children, but there is very little evidence that 

 the child is ever infected before birth. What is inherited in this case is 

 probably slight resistance to the tubercle bacillus. There is evidence 

 that almost all adult persons have been infected at one time or another 

 by this bacillus, but it has not developed far in all of them because 

 some have superior powers of resistance. Such greater or smaller re- 

 sistance, stronger or weaker build, is inherited, and while diminished 

 resistance is not the direct cause of tuberculosis it is a predisposing 

 cause. The same is probably true of many other diseases, the immedi- 

 ate causes of which are extrinsic, while only the more remote, or pre- 

 disposing causes, are hereditary. 



(c) Physiological peculiarities are inherited as well as morpholog- 

 ical ones ; indeed function and structure are only two aspects of one and 

 the same thing, namely, organization. For all morphological characters 

 there are functional correlatives, for functional characters morpholog- 

 ical expressions, and if the one is inherited so is the other. But there 

 are certain characters in which the physiological aspect is more striking 

 than the morphological one. For example, longevity is a physiological 

 character which is undoubtedly dependent upon many causes, but in the 

 case of species which differ greatly in length of life there can be little 



