74 BIOLOGY AND SOCIAL PROBLEMS 



those details of stature, color of hair, facial 

 and bodily form, inborn acts, and other traits 

 that make the child resemble the parent are 

 dependent upon this infinitely small amount 

 of livinof substance which is thus handed on 

 from parent to offspring. Although this doc- 

 trine is not without its opponents, it is so well 

 substantiated in many ways as to have gained 

 very general credence. 



One of the recent lines of evidence that 

 supports this view is that touching on sex de- 

 termination. From the most ancient times the 

 question of sex has been one of great theo- 

 retical and practical interest, and many hy- 

 potheses have been advanced and advice of 

 all kinds has been sought and given without 

 resulting in any evident control over this mat- 

 ter in either man or the lower animals. Many 

 have believed that the resting posture of the 

 body of the mother during pregnancy or the 

 character and amount of the food given her 

 would determine the sex of the developing 

 child. But all such theories were rendered 

 very improbable by the occurrence of identi- 

 cal twins, triplets, quadruplets, and so forth. 

 In man ordinary twins usually have separate 

 sets of foetal membranes and always separate 



