characteristics: relation to environment 255 



with one another and with unrelated individuals, opportu- 

 nity is presented for comparisons of the results of similar- 

 ities and diversities In genetic constitutions with those of 

 similarities and diversities in environmental influences. 



As seen in Chapter VIII, the twins derived from a single 

 egg are as a rule very closely alike in most physical respects; 

 it is this that has caused them to be called identical twins. 

 In some cases however the two differ in certain physical 

 characteristics. There appear to be two categories of causes 

 for these differences. First, there are physical differences 

 that result from the way in which the separation of the 

 twins occurred. Separation of the single egg Into two does 

 not occur In the first stages of development, as by division 

 of the one-cell stage into two. On the contrary, it occurs 

 much later, when many cells are present and the differentia- 

 tion of bodily parts has begun, and it occurs at different 

 ages in different cases. There Is reason to believe that when 

 an equal division occurs at a rather early stage, before the 

 right and left halves of the body have begun to show their 

 characteristic diversity, the two twins produced are closely 

 alike In their physical features. At a later period the right 

 and left halves of the embryo have begun to differ, in the 

 way that results as a rule in the greater strength and de- 

 velopment of the right hand and arm. At such a stage, the 

 embryo may divide Into right and left halves, with results 

 as follows. Of the two embryos so produced, one will have 

 a right limb that is already somewhat advanced In develop- 

 ment, while the left limb, formed at the plane of separa- 

 tion, must begin anew. The other embryo will be in the 

 reverse situation; it will have a left limb already somewhat 

 advanced in development, but the right limb, formed at the 

 plane of separation, must begin anew. In each case the 

 more advanced limb retains its advantage and becomes the 

 one that after birth is strongest and most used. The result 

 necessarily Is that one of the twins remains right-handed. 



