384 ON THE NUMBER OF POLAR BODIES AND [VI. 



different combinations of ancestral germ-plasm would arise, 

 and two hundred different kinds of germ-cells would be found 

 in the mature ovar}^ A still greater number of different com- 

 binations of hereditary tendencies would arise if the ' reducing 

 division ' occurred still later ; but undoubtedly the diversity in 

 the composition of the germ-plasm must be greatest of all 

 when the ' reducing division ' does not take place during the 

 period in which the germ-cells undergo multiplication, but at 

 the end of the entire course of ovarian development, and 

 separately in each full-grown mature &gg ready for embryonic 

 development. In such a case there will be as many different 

 combinations of ancestral germ-plasms as there are eggs, for, 

 as I have shown above, it is hardly conceivable that such a 

 complex body as the nuclear substance of the egg-cell — com- 

 posed of innumerable different units — would ever divide twice 

 in precisely the same manner. Every egg will therefore con- 

 tain a somewhat different combination of hereditary tendencies, 

 and thus the offspring w^hich arise from the different germ- 

 cells of the same mother can never be identical. Hence by the 

 late occurrence of the ' reducing division ' the greatest possible 

 variability in the offspring is secured. 



If my interpretation of the second polar body be accepted, it 

 is obvious that the late occurrence of the ' reducing division ' 

 is proved. At the same time we receive an explanation of 

 the advantage gained by the postponement of the reduction 

 of the germ-plasm until the end of the ovarian development 

 of the ^gg ; because the greatest possible number of individual 

 variations in the offspring are produced in this way. 



If I am not mistaken, this argument lends additional support 

 to the idea which I have previously propounded, — that the 

 most important duty of sexual reproduction is to preserve and 

 continually call forth individual variability, the foundation upon 

 which the transformation of species is built \ 



But if it be asked whether the postponement of the 'reducing 

 division ' to the end of the ovarian development of the ^gg is 

 inconsistent with the preservation of the other half of the 

 dividing nucleus, I should be inclined to reply that a ' reducing 

 division ' of the mature ^gg^ resulting in the production of two 



^ See the preceding Essa}' on ' The Significance of Sexual Repro- 

 duction in the theory of Natural Selection.' 



