LXVIJI BOWMAN LECTURE. 



at close quarters, was invisible to casual inspection and 

 the colour of such irides would undoubtedly have been 

 passed as " blue " or " grey/' meaning " devoid of stroma 

 pigment," if examined without a lens, or at a distance of 

 twelve or more inches, or in a not very well lighted room. 

 In regard to human diseases and defects I consider that, 

 in spite of, or allowing for, numerical discrepancies that 

 must occur from such causes as have been mentioned, 

 many pedigrees are, in their broad features, consistent 

 with Mendelian theory. I purposely use no stronger 

 term ; for although, as I have said, human pedigrees do 

 not, and cannot, prove the theory, we may well be inter- 

 ested in finding that some of them are at least compatible 

 with it so far as they go. Pedigrees abound in which 

 the rule, " once free always free," required by Mendelism 

 for a dominant disease is found to hold good; and others 

 occur in which the frequency of consanguineous marriage 

 and of discontinuity in transmission are consistent with a 

 recessive. It is when we come to quantities that the rela- 

 tive numbers of diseased and normal are often found to be 

 wide of the mark, sometimes far too many, sometimes not 

 nearly enough, being affected. In regard to such discre- 

 pancies we may remember, besides the hindrances to 

 complete knowledge above mentioned, that at present we 

 know very little about the indications and measure of 

 inherited liability or soil as distinguished from actual 

 disease, e. g. liability to tubercle or to mental disease ; 

 nor do we know whether in certain cases death in infancy 

 may not itself take the place of the disease that is to 

 appear later in life in the survivors."* Then, again, grant- 

 ing exact numerical segregation of unit characters, it seems 

 reasonable to expect, for man and the higher animals, 



* For a case in which D.E. x D.R. gave, in self-fertilised variegated 

 antirrhinum (snapdragon) 2 instead of 3 D. to 1 E., because the 

 remaining fraction died for want of chlorophyll diiring germination, see 

 Baur, quoted by Bateson in his Mendel's Principles of Heredity, 1909, 

 p. 253, where, under the heading " Departures from Numerical Expec- 

 tation," other facts and suggestions bearing on the subject will be 

 found. 



