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apology, if I have, first, a lengthened and uninterrupted period of 
deposit, and next, the marine fossils in any one class of that period 
well represented in it. If I am told that such representation in 
fossils is not only imperfect as regards terrestrial and soft animals, 
but also as regards molluscous shell-bearing animals, I shall go to 
issue upon the point, and, I think, prove that we ought to have, 
and do have, as good a knowledge of what were the species of 
shell-bearing molluscs which lived in the seas which produced 
many of our fossil deposits, as we have of those living at the bottom 
of our own seas at the present day ; and no one will say that that 
knowledge is very imperfect. A moment’s consideration of the re- 
spective means we have of knowing each will show the probability 
of this. The only means we have of knowing the species in our 
present seas is by dredging, or by the still more imperfect system 
of picking up those shells which may be cast ashore. Now, dredging 
is a mere scraping of a little morsel of the bottom of the sea here and 
there ; and yet, by adding up the accumulated observations made in 
various quarters, we have arrived at a most accurate knowledge of the 
inhabitants of those seas which have been examined. Some shells 
remain rare, others unique, but this does not prevent us believing in 
the accuracy of our knowledge. Compare this scraping here and there 
in the dark, with the deliberate open-day examinations which we can 
make of most geological strata ; miles upon miles of coast cliffs — 
transverse sections in ravines— and piece by piece manipulation in 
quarries and mines — and I think it must be admitted, that so far as 
that class of animals which can be preserved in deposits goes, it 
cannot be said that our knowledge of them in continuous strata is 
imperfect ; and as, therefore, we should there find the intervening 
links between older and younger species if they existed, and yet do 
not find them, the inevitable inference is that they do not exist. 
Untenable as they appear to be, however, these arguments or 
apologies have satisfied Mr Darwin, and his system of natural varia- 
tion being once admitted or held as proved, the remaining steps to 
natural selection are easy. The most essential, and one as to which 
I do not suppose there can be any difference of opinion, is founded 
on what he calls the struggle for existence. That such a struggle 
is constantly going on is familiar to us all ; but, as I neither dispute 
its existence nor its bearing (always supposing his other premises 
to be correct), I shall not make any remarks upon it, or on some of 
