NATURAL SELECTION 91 



subsequent marriage of the descendants of the first 

 pair causes an inrush of new variations, and an 

 extrusion or weakening of the inheritance from them, 

 until that inheritance is finally swamped out, and 

 appears no more. Thus Nature keeps ever fresh the 

 individualisation of her species. The ejected liquid 

 of an alarmed cuttlefish darkens the water immediately 

 in its vicinity; but what effect is produced on the 

 ocean after it has spread a mile from the spot ? 



Let us assume, for the sake of argument, that no 

 individual variation is wholly destroyed or extruded in 

 the course of successive generations, but suffers in- 

 definite dilution. In the tenth generation from an 

 ancestor his descendants will possess less than a 

 thousandth part of his variation : in the twentieth, 

 less than a millionth part. That individual variations 

 can in any way evade or elude the destructive effect 

 of marriage is inconceivable. As far as reliable 

 observation has gone, they have invariably been found 

 to fade and to ultimately perish. It is possible to say 

 up to a certain point that the law governing inheritance 

 is unknown; but this has reference only to its incalcul- 

 able action from one generation to another. No one 

 can tell how the inheritance of individual differences 

 will be portioned out among the several offspring of a 

 pair, or how many new elements of variation will 

 appear in them. But through all this individualising 

 play of Nature there is one fixed operative principle 

 that never ceases to work and produce its inevitable 

 effects, and that is the law of marriage, effective either 



