THE FACTORS OF ORGANIC EVOLUTION. 57 



prepare the way for appreciation of them, and of the relation they bear 

 to those which at present moDopolize attention. 



An observant rambler along shores, will, here and there, note 

 places where the sea has deposited things more or less similar, and 

 separated them from dissimilar things will see shingle parted from 

 sand ; larger stones sorted from smaller stones ; and will occasionally 

 discover deposits of shells more or less worn by being rolled about. 

 Sometimes the pebbles or boulders composing the shingle at one end 

 of a bay, he will find much larger than those at the other : inter- 

 mediate sizes, having small average differences, occupying the space 

 between the extremes. An example occurs, if I remember rightly, 

 some mile or two to the west of Tenby ; but the most remarkable and 

 well-known example is that afforded by the Chesil bank. Here, 

 along a shore some sixteen miles long, there is a gradual increase in 

 the sizes of the stones ; which, being at one end but mere pebbles, 

 are at the other end great boulders. In this case, then, the break- 

 ers and the undertow have effected a selection have at each place 

 left behind those stones which were too large to be readily moved, 

 while taking away others small enough to be moved easily. But 

 now, if we contemplate exclusively this selective action of the sea, 

 we overlook certain important effects which the sea simultaneously 

 works. While the stones have been differently acted upon in so far 

 that some have been left here and some carried there ; they have 

 been similarly acted upon in two allied, but distinguishable, ways. 

 By perpetually rolling them about and knocking them one against 

 another, the waves have so broken off their most prominent parts as 

 to produce in all of them more or less rounded forms ; and then, fur- 

 ther, the mutual friction of the stones simultaneously caused, has 

 smoothed their surfaces. That is to say in general terms, the actions 

 of environing agencies, so far as they have operated indiscriminately, 

 have produced in the stones a certain unity of character ; at the same 

 time that they have, by their differential effects, separated them : the 

 larger ones having withstood certain violent actions which the smaller 

 ones could not withstand. 



Similarly with other assemblages of objects which are alike in their 

 primary traits but unlike in their secondary traits. When simultane- 

 ously exposed to the same set of actions, some of these actions, rising 

 to a certain intensity, may be expected to work on particular members 

 of the assemblage changes which they cannot work in those which are 

 markedly unlike ; while others of the actions will work in all of them 

 similar changes, because of the uniform relations between these actions 

 and certain attributes common to all members of the assemblage. 

 Hence it is inferable that on living organisms, which form an assem- 

 blage of this kind, and are unceasingly exposed in common to the 

 agencies composing their inorganic environments, there must be 

 wrought two such sets of effects. There will result a universal likeness 



