360 HEEEDITY IN MAN 



experience a mild form, and others die rapidly of it. It is probable 

 that there are innately given degrees of susceptibility to all 

 diseases which are caused by the invasion of the body by small, 

 chiefly unicellular, parasites, which produce their harmful effects 

 by the secretion of toxins. The same does not apply to the 

 diseases caused by the attacks of the larger multicellular parasites, 

 such as tapeworms, which, at any rate to a large extent, produce 

 their harmful effects directly by causing lesions of the tissues. 

 As to how far we have to think of different kinds of susceptibility 

 to different diseases, or how far diseases go in classes, so that 

 susceptibility to one disease goes with susceptibility to another 

 disease and vice versa, little is known, but it is probable that in 

 some cases at least there is a linkage of susceptibilities, that, for 

 instance, susceptibility to scarlet fever is to some extent linked 

 with susceptibility to measles. We must suppose that there are 

 structural peculiarities which underlie these different suscepti- 

 bilities. Though various hypotheses have been put forward, we 

 are still ignorant as to their nature. But whatever the exact 

 nature of these structural differences, it is clear that they are 

 inherited like any other physical character. 



We have therefore to note that (a) susceptibility to specific 

 infections is inherited. If gross anomalies, such as Polydactyly 

 and syndactyly (malformations of the hand) are to be accounted 

 as diseases, then we may note that {b) gross anomalies are in- 

 herited, (c) Minor anomalies are also inherited. Under this head 

 come haemophilia, due to some peculiarity in the blood-vessels, 

 which results in their not contracting as they should and to the 

 absence of coagulating power in the blood, albuminuria, due to 

 some defect in the filtering apparatus in the kidney, albinism, 

 myopia, icthyosis, and others, all of which are clearly due to 

 structural peculiarities. Further, we may note that (d) ' other 

 conditions due, it would seem, to disturbances of metabolism, 

 underlying which may very possibly be finer anatomical variations, 

 have for long been noted as tending to be inherited ; such as 

 obesity, diabetes, gout, and chronic rheumatism '.^ Lastly (e) 

 certain nervous diseases are inherited. We may distinguish two 

 classes of nervous disease — the homeomorphic and the hetero- 

 morphic. ' In the former the offspring show the same lesions and 



* Adarai, ' Inheritance and Disease ', p. 26 (in A System of Medicine, edited by 

 Osier and McCrae). 



