are wide and shallow seas,
teeming with marine animals, and in which sediment is accumulating; now
supposing that all the hard marine animals, or rather those having hard
parts to preserve, were preserved to a future age, excepting those which
lived on rocky shores where no sediment or only sand and gravel were
accumulating, and excepting those embedded along the steeper coasts,
where only a narrow fringe of sediment was accumulating, supposing all
this, how poor a notion would a person at a future age have of the
Marine Fauna of the present day. Lyell{322} has compared the geological
series to a work of which only the few latter but not consecutive
chapters have been preserved; and out of which, it may be added, very
many leaves have been torn, the remaining ones only illustrating a
scanty portion of the Fauna of each period. On this view, the records
of anteceding ages confirm my theory; on any other they destroy it.

{320} <Note in original.> Neither highest or lowest fish (_i.e._
Myxina <?> or Lepidosiren) could be preserved in intelligible
condition in fossils.

{321} _Origin_, Ed. i. p. 290, vi. p. 425.

{322} See _Origin_, Ed. i. p. 310, vi. p. 452 for Lyell's metaphor.
I am indebted to Prof. Judd for pointing out that Darwin's version
of the metaphor is founded on the first edition of Lyell's
_Principles_, vol. I. and vol. III.; see the Essay of 1842, p. 27.

Finally, if we narrow the question into, why do we not find in some
instances every intermediate form between any two species? the answer
may well be that the average duration of each specific form (as we have
good reason to believe) is immense in years, and that the transition
could, according to my theory, be effected only by numberless small
gradations; and therefore that we should require for this end a most
perfect record, which the foregoing reasoning teaches us not to expect.
It might be thought that in a vertical section of great thickness in the
same formation some of the species ought to be found to vary in the
upper and lower parts{323}, but it may be doubted whether any formation
has gone on accumulating without any break for a period as long as the
duration of a species; and if it had done so, we should require a series
of specimens from every part. How rare must be the chance of sediment
accumulating for some 20 or 30 thousand years on the same spot{324},
with the bottom subsiding, so that a proper d