380 ANl^UAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



is notoriousl}^ not so. Evidence of some segregation is fairly clear, 

 and the deficiency of real whites may perhaps be accounted for on 

 the hypothesis of cumulative factors, though by the nature of the case 

 strict proof is not to be had. But at present I own to a preference 

 for regarding such examples as instances of imperfect segregation. 

 The series of germ-cells produced by the cross-bred consists of some 

 with no black, some with full black, and others with intermediate 

 quantities of black. No statistical tests of the condition of the 

 gametes in such cases exist, and it is likely that by choosing suitable 

 crosses all sorts of conditions may be found, ranging from the 

 simplest case of total segregation, in which there are only two forms 

 of gametes, up to those in which thete are all intermediates in various 

 proportions. This at least is what general experience of hybrid 

 products leads me to anticipate. Segregation is somehow effected by 

 the rhythms of cell-division, if such an expression may be permitted. 

 In some cases the whole factor is so easily separated that it is swept 

 out at once ; in others it is so intermixed that gametes of all degrees 

 of purity may result. That is admittedly a crude metaphor, but as 

 yet we can not substitute a better. Be all this as it may, there are 

 many signs that in human heredity phenomena of this kind are com- 

 mon, whether they indicate a multiplicity of cumulative factors or 

 imperfections in segregation. Such phenomena, however, in no way 

 detract from the essential truths that segregation occurs, and that 

 the organism can not pass on a factor which it has not itself received. 

 In human heredity we have found some examples, and I believe 

 that we shall find many more, in which the descent of factors is lim- 

 ited by sex. The classical instances are those of color blindness and 

 haemophilia. Both these conditions occur with much greater fre- 

 quenc}^ in males than in females. Of color blindness at least we know 

 that the sons of the color-blind man do not inherit it (unless the 

 mother is a transmitter) and do not transmit it to their children of 

 either sex. Some, probably all, of the daughters of the color-blind 

 father inherit the character, and though not themselves color blind, 

 they transmit it to some (probably on an average half) of their off- 

 spring of both sexes. For since these normal-sighted women have 

 only received the color blindness from one side of their parentage, 

 only half their offspring on an average can inherit it. The sons who 

 inherit the color blindness will be color blind and the inheriting 

 daughters become themselves again transmitters. Males with nor- 

 mal color vision, w^hatever their own parentage, do not have color- 

 blind descendants, unless they marry transmitting women. There 

 are points still doubtful in the interpretation, but the critical fact is 

 clear, that the germ cells of the color-blind man are of two kinds — (i) 

 those which do not carry on the affection and are destined to take part 

 in the formation of sons, and (ii) those which do carry on the color 



