384 0.V THE NUMBER OF POLAR BODIES AXD [VI. 



different combinations of ancestral germ-plasm would arise, 

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

 in the mature ovary. 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 egg 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 which 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 mj'^ 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 egg ; 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 egg 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 egg, resulting in the production of two 



' See the preceding Essay on * The Significance of Sexual Repro- 

 duction in the theory of Natural Selection.' 



