SPECIAL CASES OF SEX-INHERITANCE 237 



Identical twins are, as the name implies, extremely 

 alike. They are always of the same sex. There 

 is every presumption and some collateral evidence 

 to show that they come from one egg after fer- 

 tilization.' On ^Ene other hand, amongst ordinary 

 twins a boy and a girl, or two boys and two girls, occur 

 in the ratio expected, i.e. on the basis that their sex is 



. tit it it 



DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF OlPLOPAGI ANO 

 DUPLICATE TWINS, ILLUSTRATIVE OF THE THEORY ADVANCED IN THIS PAPER. FURTHER EX- 

 PLANATION IN THE TEXT. 



FIG. 110. Diagram showing different types of union of double monster 

 (After Wilder.) 



not determined by a common external or internal 

 cause. Since fraternal twins and identical twins show 

 these relations at birth and from the fact that they 

 have been in both cases subjected to the same condi- 

 tions, it follows with great probability that sex in 

 such cases is determined before or at the time of 

 fertilization. 



This conclusion finds strong support from the condi- 



