256 GENETICS 



while the other becomes left-handed. This situation Is not 

 uncommonly found in identical twins. Similar differences in 

 other unsymmetrical parts may be produced in the same 

 way: for example in the whorls of hair on the head. In such 

 respects the two members of a pair often show "mirror- 

 imaging," presumably due in each case to the fact that the 

 original embryo had already become a little unsymmetrical 

 before division occurred. 



From this same situation another condition often ob- 

 served is believed to result. In the embryo before division, 

 one side (usually the right) will be a little in advance of 

 the other. After division, the twin derived from the right 

 half continues to retain this advantage, so that it may be 

 more vigorous than the one derived from the left half. 

 Such differences in vigor are not rare in twins. Apparently 

 they may be accompanied by a psychological difference; the 

 more vigorous twin assumes the leadership in their lives 

 together. This reacts farther on temperament and charac- 

 ter. How far psychological and temperamental differences 

 so produced may go is uncertain. 



It appears that in some cases division of an embryo may 

 be unequal, so that for this reason also one twin may be 

 more vigorous than the other, with resulting psychological 

 differences. 



Diversities produced In the ways above described are 

 different in origin and type from those commonly called 

 environmental. Under the latter term are commonly 

 grouped diversities resulting from the different conditions 

 under which individuals have lived. Since the extent to 

 which differences due to the time and method of division 

 may go, and the kind of characteristics that they may affect 

 are uncertain, it Is often not possible to distinguish these 

 certainly from diversities due to differences of environment 

 after division has occurred. 



Belonging to one or the other class, there are sometimes 



