What did Bentham mean by "outworks" in an address to the Linnean Society?

Question

In Life and Letters of Charles Darwin, volume 2, the editor was talking about Mr. Bentham:

[The next letter refers to Mr. Bentham's presidential address to the Linnean Society (May 25, 1863). Mr. Bentham does not yield to the new theory of Evolution, "cannot surrender at discretion as long as many important outworks remain contestable." But he shows that the great body of scientific opinion is flowing in the direction of belief.

I can't find in the dictionaries a suitable meaning for "outworks" in this context.

Answer 1

outwork, n. 1.a. […] any detached or advanced work forming part of the defence of a place; an outer defence

_{Oxford English Dictionary.}

So Bentham is giving us a metaphor in which his belief in the fixity of species is like a fortress, the theory of evolution by natural selection is like a besieging army, and the various points of contention are like outworks, which have to be captured before the fortress can fall.

Bentham was eventually convinced by Darwin’s theory:

It was reserved for the publication of the ‘Origin of Species’ in 1859 to mark out a practicable path by which the higher summits might be attained. The doctrine of evolution of species, according to laws originally fixed, instead of arbitrary intervention upon each and every occasion, was in this remarkable work clearly traced out, supported by powerful arguments, and founded upon facts and observations the accuracy of which no one could doubt; and a way was thus opened up to a pinnacle, which in a wonderful degree enlarged the range of vision of those who had the courage to follow its propounder up the giddy height.

_{George Bentham (1874). ‘On the Recent Progress and Present State of Systematic Botany. In The American Journal of Science and Arts 9:52, p. 291.}