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.