350 THE ZOOLOGIST. 



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 there 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 inter- 

 mixed that gametes of all degrees of purity may result. That is 

 admittedly a crude metaphor, but as yet we cannot substitute a 

 better. Be all this as it may, there are many signs that in human 

 heredity phenomena of this kind are common, 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 cannot 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 

 limited by sex. The classical instances are those of colour-blindness 

 and hemophilia. Both these conditions occur with much greater 

 frequency in males than in females. Of colour-blindness at least we 

 know that the sons of the colour-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 colour- 

 blind father inherit the character, and though not themselves colour- 

 blind, they transmit it to some (probably, on an average, half) of 

 their offspring of both sexes. For since these normal-sighted women 

 have only received the colour-blindness from one side of their 

 parentage, only half their offspring, on an average, can inherit it. 

 The sons who inherit the colour-blindness will be colour-blind, and 

 the inheriting daughters become themselves again transmitters. 

 Males with normal colour-vision, whatever their own parentage, do 

 not have colour-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 colour-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 colour-blindness and are destined to form 

 daughters. There is evidence that the ova also are similarly pre- 

 destined to form one or other of the sexes, but to discuss the whole 

 question of sex-determination is beyond my present scope. The 

 descent of these sex-limited affections nevertheless calls for mention 

 here, because it is an admirable illustration of factorial predestination. 

 It moreover exemplifies that parental polarity of the zygote to which 



