44 THE FACTORS OP ORGANIC EVOLUTION. 



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 in 

 crease in the sizes of the stones; which, being at one end 

 but mere pebbles, are at the other end immense boulders. 

 In this case, then, the breakers and the undertow have 

 effected a selection have at each place left behind those 

 &quot;tones 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, further, 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 indiscri 

 minately, have produced in the stones a certain unity of 

 character; at the same time that they have, by their 

 differential effects, separated them : the Jarger 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 simultaneously 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 



