IN THE THEOEY OF NATURAL SELECTION. 331 



differ at any rate in strength. But this is by no means always 

 the case, for even in horses the reverse may occur. Thus it is 

 stated that certain female race-horses have always transmitted 

 their own peculiarities, while others allowed those of the stallion 

 to preponderate. 



In the human species the influence of the mother preponderates 

 quite as often as that of the father, although in many families most 

 of the children may take after either parent. There is nevertheless 

 hardly any large family in which all the children take after the 

 same parent. If we now try to explain the preponderating in- 

 fluence of one parent by the supposition of a greater strength in 

 hereditary power, without first inquiring after some deeper cause, 

 I think the only conclusion warranted by the facts before us is 

 that this power is rarely or never equal in both of the conjugating 

 germ-cells, but that even within the same species, sometimes the 

 male and sometimes the female is the stronger, and that the strength 

 may even vary in the different offspring of the same individuals, 

 as we so frequently see in human families. The egg-cells of the 

 same mother which ripen one after the other, and also the sperm- 

 cells of the same father, must therefore present variations in the 

 strength of their hereditary power. It is then hardly to be wondered 

 at that the relative hereditary power of the germ-cells in different 

 species should vary, although we cannot as yet understand why 

 this should be the case. 



It would not be very difficult to render these facts intelligible 

 in a general way by an appeal to physiological principles. The 

 quantity of germ-plasm contained in a germ-cell is very minute, 

 and together with the idioplasms of the various ontogenetic 

 stages to which it gives rise, it must be continually increased by 

 assimilation during the development of the organism. If now this 

 power of assimilation varied in intensity, a relatively rapid growth 

 of the idioplasm derived from one of the parents would ensue, 

 and with it the preponderance of the hereditary tendencies of 

 the parent in question. Now, it is obvious that no two cells of 

 the same kind are entirely identical, and hence there must be 

 differences in their powers of assimilation. Thus the varying 

 hereditary powers of the egg-cells produced from the same ovary 

 become explicable, and still more easily the varying powers of the 

 germ-cells produced in the ovaries or testes of different individuals 



