TRANSACTIONS OF SECTION 11. 763 



generations, and perhaps even in a seventh, and was, as a rule, transmitted 

 alternately from the males to the females of the family' — 



F? 



I 

 M 



I 



F 



I 



M 



i 



F 

 I 



I I I I I 



M M M F F 



I I 



M M 



In this and the following diagrnms M stands for male, P for female, whilst the black type (^M oi F) 

 marks the iu dividual or generation iu wliicb the variation occurred. 



Another noticeable deformity which is known to be hereditary in some families, 

 and which may be familiar to some of my auditors, is that of imperfect development 

 of the upper lip and roof of the mouth, technically known as hare-lip and cleft palate. 



These examples illustrate what may be called the coarser kinds of hereditary 

 deformity, where the redundancies or defects in parts of the body are so f^ross as 

 at once to attract attention. But modifications or variations in structure that can 

 be transmitted from parent to offspring are by no means limited to changes which 

 can be detected by the naked eye. They are sometimes so minute as to be deter- 

 mined rather by the modifications which they occasion in the function of the organ 

 than by the ready recognition of structural variations. One of the most interesting of 

 these is the affection known as Daltonism, or colour-blindness, which has distinctly 

 been shown to be hereditary, and which is due, apparently in the majority of 

 cases, to a defect in the development of the retina, or of the nerve of sight which 

 ends in it, though in some instances they may be occasioned by defective develop- 

 ment of the brain itself. Dr. Horner has related a most interesting family history,* 

 in which the colour-blindness was traced through seven generations. In this 

 family the males were the persons atiected, though the peculiarity was transmitted 

 through the females, who themselves remained unaffected. The family tree 

 showed that in the sixth generation seven mothers had children. Their sons, 

 collectively nine in number, were all colour-blind with the exception of one son, 

 while none of their nine daughters showed the hereditaiy defect. 



M 



F F 



I I 



M M 



I 



F 



II II 



MM MM 



I I 



I I I I I II 



F F F F F F F 



I I I I I II 



II ! I I I I I I I I I I I I I I 

 MM M F Ji M F F M M M F F M f f f 



' Joum. Anat. and Phys., vol. xviii., page 4G3. 



' Cited in Die Allgemcine Pathologic, by Dr. Edwin Klebs, Jena, 1887. 



