GENESIS, HEREDITY, AND VARIATION. 289 



au organism, acts, by its polar forces, on all the other units, 

 and is re-acted on by them. When, too, we learn that 

 glass has its molecular constitution changed by light, and 

 that substances so rigid and stable as metals, have their 

 atoms re-arranged by forces radiated in the dark from 

 adjacent objects ; we are obliged to conclude that the ex- 

 cessively-unstable units of which organisms are built, must be 

 sensitive in a transcendant degree, to all the forces pervading 

 the organisms composed of them must be tending ever to 

 re-adjust, not only their relative positions, but their molecular 

 structures, into equilibrium with these forces. Hence, if ag- 

 gregates of the same species are differently conditioned, and 

 re-act differently on their component units, their component 

 units will be rendered somewhat different ; and they will 

 become the more different the more widely the re -actions 

 of the aggregates upon them differ, and the greater the num- 

 ber of generations through which these different re-actions of 

 the aggregates upon them are continued. 



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 so 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 

 depends on their derivation from different individuals. It 

 explains the fact 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 fertility and vigour. 



