ENVIRONMENT NOT SOLE FACTOR 519 



those produced by the division of a single fertilized egg. 

 It is probable that the latter mode of origin accounts 

 for all cases of so-called " identical " twins, that is, twins 

 who are almost exact duplicates of each other. Identical 

 twins are always of the same se^, while " fraternal " twins 

 may be either of the same or opposite sex. It is believed 

 that it is the presence of identical twins in the groups 

 which have been measured and tested that is largely re- 

 sponsible for the greater resemblance of twin-pairs as 

 compared with ordinary sib-pairs. However, the ques- 

 tion is still in dispute, largely owing to the impossibility 

 of our knowing to a certainty which twins are of single- 

 egg or of two-egg origin. 



Family Resemblance not Wholly Accounted for 

 BY Environment. — One is naturally tempted to ex- 

 plain the mental resemblance of parent and child, and 

 especially that of sib and sib, or twin and twin, as due 

 to similarity of environment and training. There are 

 several facts, however, which render this theory unten- 

 able. (1) The resemblance in mental traits is about as 

 great as in physical traits, and it is known that the latter 

 are not determined chiefly by environment. (2) The 

 resemblance of twins in mental tests which have novel 

 content is as great as in tests whose content is more sub- 

 ject to the influence of training. (3) Older twins have 

 no higher coefficient of mental resemblance than younger 

 twins. If the resemblance were due chiefly to environ- 

 ment, then it should become greater the longer that en- 

 vironment operates. It does not. (4) Galton found 

 twins who had been subjected to very different environ- 

 ment who were nevertheless extremely similar. (5) It 

 is a matter of common observation that sibs who differ 

 greatly from each other in early childhood may continue 

 to differ, notwithstanding the similar environment they 

 enjoy throughout their period of plasticity. (6) Galton 

 found that the adopted sons of popes were far less likely 

 to become eminent than the real sons of great men who 



