12 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



Second. If the human germ-plasm is continually mutating to 

 produce one or another kind of specific defect, this will increase 

 the chance for any recurrent defect to finally establish itself. 

 That particular mutations do recur in other animals is now 

 abundantly established by evidence that comes from several 

 sources. 



Third. There is a growing impression that a good deal of 

 feeblemindedness and insanity are environmental rather than he- 

 reditary traits; poverty, malnutrition, and especially syphilis are 

 said to play a considerable role in their production. It is unsafe 

 therefore to conclude that the human germ-plasm is as badly con- 

 taminated as some pessimists seem to think. 



If we turn now more directly to special kinds of human in- 

 heritance we shall find a great deal of evidence showing that the 

 same laws of inheritance that hold for animals and for plants ap- 

 ply to man. It would be surprising if this were not the case. 



On the other hand, when we scrutinize the pedigrees that have 

 been published to illustrate heredity in man, we shall find many of 

 them very unsatisfactory in two main respects, (i) The num- 

 ber of offspring in a family is usually too small to serve as a 

 sample of the germ-plasm of the parents. (2) Therefore, since 

 recourse must be had to many families for sufficient data, it is 

 essential that the diagnosis of the defects of the parents and of the 

 children is correct. A single mistake may throw the result into 

 confusion. In cases where the defect is structural, a correct clas- 

 sification may be possible, but in other cases, especially where psy- 

 chological defects are involved, the diagnosis is difficult and the 

 results, in consequence, less certain. Often the best that we can 

 do in the case of man is to try to find the simplest Mendel ian for- 

 mula to which the evidence will fit. If one factor-difference will 

 not suffice, then two must be tried; if two will not do, then three 

 must be tried, etc. Now I need hardly point out that we can ex- 

 plain almost anything if we are allowed enough factors. It is, at 

 best, a dangerous practice, one to be used only with great caution 

 and the conclusion stated as provisional and checked in every pos- 

 sible way. 



