232 Heredity and Pathology [ch. 



The simplest kind of sex-limitation is illustrated in 

 Fig. 33. No example is yet known in man which exactly 

 follows this system, though perhaps haemophilia and night- 

 blindness may approximate to it. * Colour- Blindness, as 

 stated above (pp. 172 and 195), shows remarkable com- 

 plications in the fact that not merely the development of 

 the peculiarity but also the actual transmission of the 

 responsible factor is affected by sex. Fig. 34 gives a ten- 

 tative representation of the system which is perhaps followed 

 by such a condition, but it is given rather as a hint to 

 collectors of evidence than as a positive statement of as- 

 certained fact. 



Recessive characters will be recognized by the fact that 

 they may appear in the children of parents not exhibiting 

 such characters, and especially among children born of 

 consanguineous marriages. Complete proof of the recessive 

 nature of a characteristic will only be obtained by evidence 

 that all the children of affected parents exhibit the charac- 

 teristic. 



In all these descents occasional exceptions are to be 

 expected, which in the light of present knowledge cannot 

 be interpreted; but when the habitual course of the in- 

 heritance has been determined, an inquiry into the nature 

 of these exceptions may perhaps be undertaken with 

 success. 



A correct knowledge of the system which a hereditary 

 disease follows in the course of its descent will, it may be 

 anticipated, contribute to a proper understanding of the 

 pathology of these affections, and thus make a definite 

 advance in the general study of the physiology of disease. 

 If, for example, a disease descends through the affected 

 persons, as a dominant, we may feel every confidence that 

 the condition is caused by the operation of a factor or 

 element added to the usual ingredients of the body. In 

 such cases there is something present, probably a definite 

 chemical substance, which has the power of producing the 

 affection. Thus the abortion of the digits, or the inability 

 to distinguish certain colours, must be due to such added 

 factors. On the contrary, when the disease is recessive we 

 recognize that its appearance is due to the absence of some 

 ingredient which is present in the normal body. So, for 



