340 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



influence of the ass is stronger in both cases, only predo- 

 minating to a less extent when the ass is the female parent ; 

 and I will for the sake of brevity accept Brooks' opinion that in 

 these cases the influence of the father is greater than that of the 

 mother. Were this so in all cross-breeding between different 

 species and in all cases of normal fertilization, we should be 

 compelled to conclude that the influences of the male and 

 female germ-plasms upon the offspring 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 pre- 

 ponderate. 



In the human species the influence of the mother prepon- 

 derates quite as often as that of the father, although in man}^ 

 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 influence of one parent by the supposition 

 of a greater strength in hereditary power, without first in- 

 quiring 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 some- 

 times the female is the stronger, and that the strength maj' 

 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, 



