288 C. G. CAMPBELL 



can only give the briefest summary of Bickerton's findings.. From studies 

 extending over a personal experience of forty years, Bickerton shows that 

 there is a hereditary factor in myopia, or short-sightedness, in detachment 

 of the retina, astygmatism, long-sightedness, strabismus, nystagmus, ptosis, 

 opthalmoplegia, microphthalmia, retinal atrophy, optic nerve atrophy, 

 optic neuritis, aniridia, or absence of iris, congenital and senile cataract, 

 ectopia lentis, glaucoma, blue sclerotics, glioma, night blindness, day blind- 

 ness, and color blindness. Most of these affections can lead to complete 

 blindness. Bickerton shows beyond any question that there is a hereditary 

 factor in the occurrence of all of them, and that in most cases the recurrence 

 in progeny is high. Hence the contention that blindness seldom or never 

 has a hereditary cause is a mischievous and dangerous untruth. 



Davenport has proved beyond doubt that otosclerosis, a leading cause of 

 deafness, is clearly hereditary in its origin. And Blakeslee has shown that 

 deficiencies in the taste function likewise have a hereditary origin. 



In any species a high general resistance to disease is of prime importance 

 to its survival. There has been much discussion of the inheritance of dis- 

 ease as to whether a particular disease could be directly inherited, or perhaps 

 only a certain tendency to it. While it is desirable to make this discrimina- 

 tion in regard to a disease and to discover the immediate causes of its occur- 

 rence, the importance of the heritable factor can be readily estimated by the 

 frequency in which a disease recurs in family strains. A great number of 

 family studies have been made and family charts compiled showing the 

 incidence not only of physical abnormalities but of particular diseases 

 occurring in family strains. It is invariably discovered that this incidence 

 is far higher in particular family strains than it is in the population at large, 

 and a Mendelian ratio can often be traced. It would seem idle and irra- 

 tional to assert that this fact was unconnected with the germ-plasm and the 

 genes which are inherited in a family strain, and which determine the phys- 

 ical constitution of individuals. 



One might ask the question why some people contract a particular disease 

 and others do not, and receive the answer that it was because such in- 

 dividuals would be the more exposed to such a disease. All observation 

 however goes to show the more correct answer to be that some are the more 

 susceptible to such a disease. In other words they have a congenital con- 

 stitutional weakness which the more easily admits of the invasion of such a 

 disease. 



But let us take an example of where a whole community would be ex- 

 posed in an equal degree to a disease. Davenport made a study of a com- 

 munity in a valley in the Blue Ridge Mountains in which goitre was highly 



