at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this
history we possess the last volume alone, relating only to two or
three countries. Of this volume, only here and there a short
chapter has been preserved; and of each page, only here and there a
few lines. Each word of the slowly-changing language, in which the
history is supposed to be written, being more or less different in
the interrupted succession of chapters, may represent the
apparently abruptly changed forms of life, entombed in our
consecutive, but widely separated formations." Professor Judd has
been good enough to point out to me, that Darwin's metaphor is
founded on the comparison of geology to history in Ch. i. of the
_Principles of Geology_, Ed. i. 1830, vol. i. pp. 1-4. Professor
Judd has also called my attention to another
passage,--_Principles_, Ed. i. 1833, vol. iii. p. 33, when Lyell
imagines an historian examining "two buried cities at the foot of
Vesuvius, immediately superimposed upon each other." The historian
would discover that the inhabitants of the lower town were Greeks
while those of the upper one were Italians. But he would be wrong
in supposing that there had been a sudden change from the Greek to
the Italian language in Campania. I think it is clear that Darwin's
metaphor is partly taken from this passage. See for instance (in
the above passage from the _Origin_) such phrases as "history ...
written in a changing dialect"--"apparently abruptly changed forms
of life." The passage within [] in the above paragraph:--"Lyell's
views as far as they go &c.," no doubt refers, as Professor Judd
points out, to Lyell not going so far as Darwin on the question of
the imperfection of the geological record.

_Extermination._ We have seen that in later periods the organisms have
disappeared by degrees and [perhaps] probably by degrees in earlier, and
I have said our theory requires it. As many naturalists seem to think
extermination a most mysterious circumstance{121} and call in
astonishing agencies, it is well to recall what we have shown concerning
the struggle of nature. An exterminating agency is at work with every
organism: we scarcely see it: if robins would increase to thousands in
ten years how severe must the process be. How imperceptible a s