VIII] HEREDITY IN MAN 113 



with confidence that among the offspring of a parent 

 who has congenital cataract or abnormally jointed 

 fingers, about one-half will be similarly affected, and 

 there is no hope in such a case that the severity 

 of the affection will diminish in later generations. 

 Where the disease depends on several factors, it 

 may perhaps be eliminated by repeated marriage 

 with untainted stock, but in such cases as cataract 

 or colour-blindness there is no hope of this. 



It is commonly supposed that inherited disease 

 arises largely from the cumulative effect of bad 

 conditions, drink and the like, but it has been seen 

 how doubtful it is whether the effects of such things 

 are really transmitted, and in any case it can be 

 proved that in comparison with the germinal con- 

 stitution, the effects of environment are relatively 

 insignificant. Galton was one of the first to illustrate 

 this by the study of twins. Human twins are of two 

 sorts ; in one case they arise by the simultaneous 

 development of two ova, as in the litters of lower 

 animals, and then they are no more alike than other 

 children of the same parents, and may be of different 

 sexes. Twins of the second kind are probably pro- 

 duced by division of one ovum, and are then of the 

 same sex and so alike as to be called 'identical.' 

 Such identical' twins remain through life, despite 

 differences of environment, more like one another 

 than successively born brothers commonly are, even 

 D. 8 



