at changes of
this class pass entirely unrecognized; for, as we shall see, Mr. Darwin
takes cognizance of certain secondary and special ones. But the effects
which are not taken into account, are those primary and universal
effects which give certain fundamental characters to all organisms.
Contemplation of an analogy will best prepare the way for appreciation
of them, and of the relation they bear to those which at present
monopolize 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: intermediate 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 immense
boulders. In this case, then, the breakers 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, 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
indiscriminately, have produced in the stones a certain unity of
character; at the same time that they have, by their differential
effects, separated them: