LXVI BOWMAN LEOTCRE. 



sion on crossing," " epistatic " and " liypostatic " factors, 

 and the complex results that flow from them, results in 

 which the actual and the expected numbers are often so 

 strikingly near. 



Whether the Mendelian theory, or any one of the 

 current doctrines of heredity, contains the whole truth 

 is perhaps doubtful ; but we may rest assured that sooner 

 or later a ground will be discovered upon which the 

 advocates of the various theories can meet in common. 

 Meantime, I conceive that our contribution to the problem, 

 as students of the natural history of living man, should 

 consist in the collection, classification and analysis of 

 fresh pedigrees of disease or defect wherever we can find 

 them. 



The Mendelian theory in its simple form is so precise, 

 and in regard to a number of unit characters in certain 

 plants and animals its expectation has been found to fit so 

 nearly with experimental results, that no surprise can be 

 felt at the attraction it has for workers in human heredity. 

 But, founded as the theory is upon a strictly quantitative 

 conception, it would certainly never have been formulated 

 from data afforded by human disease alone, and this 

 for several reasons. Thus it is difficult and often impos- 

 sible to get a record of all the maternal conceptions ; and 

 even then we do not know how many of the miscarriages 

 and stillbirths, and but rarely how many of those born 

 alive but dying in infancy, would have been affected."* 

 Again, when dealing with a condition that comes on many 

 years after birth the record is incomplete unless all can 

 be followed quite up to the susceptible age. Then it is, 

 to say the least, probable that in some cases the disease 

 which exists potentially may never appear for lack of some 

 agent or influence called for want of a better name an 

 excitant or stimulant that is necessary to complete it, 

 e. g. Fig. 38, retinitis pigmentosa probably brought out 



* We can at present only assume that had these immaturities and 

 early deaths survived they would have suffered in the same proportion 

 as those who lived. This assumption, however, may be unwarranted. 



