The Theory of Natural Selection. 283 
expanding an illustration which, I belicve, was first 
used by Professor Huxley. If, when the tide is out, we 
see lying upon the shore a long line of detached sea- 
weed, marking the level which is reached by full tide, 
we should be free to conclude that the separation of 
the sea-weed from the sand and the stones was due 
to the intelligent work of some one who intended to 
collect the sea-weed for manure, or for any other pur- 
pose. But, on the other hand, we might explain the 
fact by a purely physical cause—namely, the separa- 
tion by the sea-waves of the sea-weed from the sand 
and stones, in virtue of its lower specific gravity. Now, 
thus far the fact would be explained equally well by 
either hypothesis; and this fact would be the fact of 
selection. But whether we yielded our assent to the 
one explanation or to the other would depend upon a 
due consideration of all collateral circumstances. The 
sea-weed might not be of a kind that is of any use to 
man; there might be too great a quantity of it to 
admit of our supposing that it had been collected by 
man; the fact that it was all deposited on the high- 
water-mark would in itself be highly suggestive of the 
agency of the sea; and so forth. Thus, in such a case 
any reasonable observer would decide in favour of the 
physical explanation, or against the teleological one. 
Now the question whether organic evolution has 
been caused by physical agencies or by intelligent 
design is in precisely the same predicament. There 
can be no logical doubt that, theoretically at all events, 
the physical agencies which the present chapter is con- 
cerned with, and which are conveniently summed up in 
the term natural selection, are as competent to produce 
these so-called mechanical contrivances, and the other 
