GENESIS, HEREDITY, AND VARIATION. 353 



If, then, unlikenesses of function among individuals of the 

 same species, produce unlikenesses between the physiological 

 units of one individual and those of another, it becomes com- 

 prehensible that when groups of units derived from two indi- 

 viduals are united, the group formed will be more unstable 

 than either of the groups was before their union. The mixed 

 units will be less able to resist those re-distributing forces 

 which cause evolution; and may thus have restored to them 

 the capacity for development which they had lost. 



This view harmonizes with the conclusion, which we saw 

 reason to draw, that fertilization does not depend on any 

 intrinsic peculiarities of sperm-cells and germ-cells, but de- 

 pends on their derivation from different individuals. It 

 explains the facts that nearly-related individuals are less likely 

 to have offspring than others, and that their offspring, when 

 they have them, are frequently feeble. And it gives us a key 

 to the converse fact that the crossing of varieties results in 

 unusual vigour. 



Bearing in mind that the slightly-different orders of phy- 

 siological units which an organism inherits from its parents, 

 are subject to the same set of forces, and that when the 

 organism is fully developed this set of forces, becoming con- 

 stant, tends slowly to re-mould the two orders of units into 

 the same form; we see how it happens that self-fertilization 

 becomes impossible in the higher organisms, while it remains 

 possible in the lower organisms. In long-lived creatures which 

 have tolerably-definite limits of growth, this assimilation of 

 the somewhat-unlike physiological units is liable to go on to 

 an appreciable extent; whereas in organisms which do not 

 continuously subject their component units to constant forces, 

 there will be much less of this assimilation. And where the 

 assimilation is not considerable, the segregation of mixed 

 units may cause the sperm-cells and germ-x^ells developed in 

 the same individual, to be sufBciently different to produce, by 

 their union, fertile germs; and several generations of self- 

 fertilizing descendants may succeed one another, before the 



