50 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



out function. But if the spermatozoa are thus in- 

 efficient in the production of males, how is the species 

 of phylloxerans maintained? The answer is that 

 the females reproduce without fertilization (that is, 

 by parthenogenesis) for several generations, always, 

 however, producing females. Ultimately the par- 

 thenogenetic females produce males. Very signifi- 

 cantly, and consistently with what has already been 

 said, this formation of males is attended by the 

 loss of a chromosome from each egg destined to 

 make a male. A similar explanation probably 

 applies to the bees, although this has not yet been 

 proved. These striking observations in animals 

 have a remarkable analogy in some flowering plants. 

 Correns has shown in cases of plants where the sexes 

 are separate that half the pollen grains are female 

 makers and half are male makers. These are the 

 most obvious and vital facts at present known. 

 There is indirect evidence which makes it probable 

 that sex is predetermined in man and in the higher 

 animals, as in the insects and related animals. The 

 microscopical evidence of this is now lacking, but 

 will doubtless be forthcoming. 



These facts tend to strengthen our fatalistic beliefs. 

 That sex is determined at the instant of conception 

 is in itself a remarkable thing. And if this deter- 

 mination depends on the operation of one or more 

 specialized chromosomes, this result is a highly 

 significant contribution to the mechanistic basis 

 of vital phenomena. 



