VARIATION AND HEREDITY IN TWINS 169 



twins with identical brains should have identical mental 

 equijiment at birth, but further mental development 

 would depend on training and environment. Many 

 accounts have been given of the remarkable unity of 

 thought and action of dupHcate twins. They have 

 been described as speaking in unison with the same 

 inflection, as having the same dreams, etc. Even 

 more remarkable are stories to the effect that if one 

 twin becomes ill the other does also, and that an injury 

 to one twin is felt by the other. Such anecdotes, 

 though common, are probably without foundation. 

 More credible are stories that one twin got two baths 

 and the other none, or that one twin was spanked 

 for the other's misdeeds. 



THE COMPARATIVE POTENCY OF HEREDITY 

 AND ENVIRONMENT 



To what extent and within what limits are the 

 definitive characters of the individual determined 

 at the time of fertilization, and in how far are the 

 minutiae of organic structure to be considered as the 

 product of individual variability beyond the limits of 

 hereditary control? This very fundamental question 

 has been raised under various guises for many years. 

 It is the old problem as to the relative potency of 

 heredity and environment in development, or that of 

 predetermination versus epigenesis. 



As long ago as 1870 Galton previsioned the impor- 

 tance of the study of twins as material probably adapted 

 to a solution of the problem. His views are very 

 clearly stated in his paper "The History of Twins, as 

 a Criterion of the Relative Power of Nature and 



