epth might be preserved for
any one species to continue living: what an amount of subsidence would
be thus required, and this subsidence must not destroy the source whence
the sediment continued to be derived. In the case of terrestrial
animals, what chance is there when the present time is become a
pleistocene formation (at an earlier period than this, sufficient
elevation to expose marine beds could not be expected), what chance is
there that future geologists will make out the innumerable transitional
sub-varieties, through which the short-horned and long-horned cattle
(so different in shape of body) have been derived from the same parent
stock{325}? Yet this transition has been effected in _the same country_,
and in a far _shorter time_, than would be probable in a wild state,
both contingencies highly favourable for the future hypothetical
geologists being enabled to trace the variation.

{323} See _More Letters_, vol. I. pp. 344-7, for Darwin's interest
in the celebrated observations of Hilgendorf and Hyatt.

{324} This corresponds partly to _Origin_, Ed. i. p. 294, vi. p.
431.

{325} _Origin_, Ed. i. p. 299, vi. p. 437.

CHAPTER V

GRADUAL APPEARANCE AND DISAPPEARANCE OF SPECIES{326}

{326} This chapter corresponds to ch. X of _Origin_, Ed. i., vi.
ch. XI, "On the geological succession of organic beings."

In the Tertiary system, in the last uplifted beds, we find all the
species recent and living in the immediate vicinity; in rather older
beds we find only recent species, but some not living in the immediate
vicinity{327}; we then find beds with two or three or a few more extinct
or very rare species; then considerably more extinct species, but with
gaps in the regular increase; and finally we have beds with only two or
three or not one living species. Most geologists believe that the gaps
in the percentage, that is the sudden increments, in the number of the
extinct species in the stages of the Tertiary system are due to the
imperfection of the geological record. Hence we are led to believe that
the species in the Tertiary system have been gradually introduced; and
from analogy to carry on the same view to the Secondary formations. In
these latter, however, entire groups of species generally come in
abruptly; but this would naturally result, if, as argued in the
foregoing chapter, these Secondary deposits are separated by wide
epochs. Moreover it is important t