92 THE FIRST PRINCIPLES OF HEREDITY 



tend to produce males, while, at the same time, the females, 

 being superabundant in number, will have less chance of 

 being fertilized, and therefore, if fertilized, will have old 

 ova, also tending towards a preponderance of the male sex. 

 We must further point out that identical twins, which 

 arise from a single ovum, are always of the same sex. It 

 seems, therefore, a very probable conclusion to assume, 

 from the various reasons adduced, that the sex is already 

 fixed in the fertilized ovum, in some cases, perhaps, even 

 before fertilization, though environmental conditions may 

 in other cases have the final determining influence. In fact, 

 as is only likely to be expected, the different factors men- 

 tioned have a varying degree of influence in the different 

 classes of organisms. 



