RELATION OF GENES TO CHARACTERISTICS 201 



size. Their teeth are alike, even to the irregularities. Their 

 stature and weight are very similar. Their finger prints are 

 as much alike as those of the two hands of the same individ- 

 ual. 



In all these respects the two-egg twins are often very 

 different. They are no more alike than any other two mem- 

 bers of the same family. 



Identical twins are also alike in weaknesses, in tendencies 

 to certain diseases. If one has tuberculosis or other consti- 

 tutional disease at a certain time of life, the other one usu- 

 ally has it also at about the same time, even though the , 

 twins live apart. But two-egg twins do not show these re- 

 semblances. 



All this demonstrates that all these physical characteris- 

 tics in man depend on genes. They are alike in two individ- 

 uals whose genes are alike, diverse in two individuals whose 

 genes are diverse. 



One-egg twins at times show certain physical differences, 

 resulting either from certain features of the process of 

 splitting the original egg from which the two were pro- 

 duced, or from the fact that one of the twins has been in- 

 jured by accident or disease. From the former cause arise 

 the facts that ( i ) sometimes one of the twins is right- 

 handed, the other left-handed, and (2) one is sometimes 

 more vigorous than the other. If the splitting of the original 

 single egg occurs very early, before the right and left sides 

 have become diverse, the two twins are alike in "handed- 

 ness" and in vigor; if the splitting occurs later they may be 

 diverse in these respects. The effects of diversity of envi- 

 ronment on twins we take up in a later chapter. 



Mental Characteristics in Twins: In a number of cases 

 the two twins from a single egg have been separated while 

 very young — before the age of one or two years, and have 

 lived apart afterward. In such cases the genes of the two 

 individuals are alike, while their environments have been 



