112 THE FIRST PRINCIPLES OF HEREDITY 



II.— SPECIAL DISEASES. 



As we are dealing with the subject of diseases only from 

 the biological point of view, it would be useless to give an 

 exhaustive list of diseases, inherited and acquired. We 

 shall only mention such diseases as bear on the problem 

 under discussion and are of interest to the general reader. 



(a) Predispositions. 



We have already pointed out that in a certain number of 

 diseases what is inherited is not the disease itself, but a pre- 

 disposition towards it. To this class belong tuberculosis, 

 gout, rheumatism, etc., where, given a constitutional weak- 

 ness towards such disorder, the disease is easily brought 

 into manifestation by some additional external factor ; in 

 the case of consumption by the infection with the tubercle 

 bacillus, in gout and rheumatism by other harmful 

 agencies. Similarly, it can be asserted with reason that 

 insanity and other nervous disorders are generally due in 

 the last instance, not to shock or other external disturbing 

 influences, as so often asserted, but to the inherent unstable 

 equilibrium of the nervous S37stem of the individuals 

 affected. Unfortunately, such nervous disposition may run 

 in families, and is to a great extent inheritable, though it 

 may assume in the various members of the family different 

 forms. Hysteria, epilepsy, mania, imbecility, etc., may all 

 be signs of the same degeneracj/, which fact furnishes the 

 best proof that what is inherited is not a specific disease, 

 but a general tendency towards neuroses. 



{h) Inherited Diseases. 



Of inherited diseases we mention only two, which have a 

 special interest from the curious phenomena connected 

 with them. One is colour-blindness, or Daltonism (called 

 so after the famous chemist Dalton, who himself suffered 



