Greg offered a talk to prove "the lower animals possess every faculty & propensity of the human mind".
In 1868 he responded to Darwin's On the Origin of Species with an article in Fraser's Magazine, "On the failure of 'Natural Selection' in the case of Man".
In this he argues that natural selection no longer operates within civilised societies, endorsing eugenicist ideas to counter this fact.
Besides contributions to periodicals he produced several volumes of essays on political and social philosophy.
They represent a reaction from the high hopes of the author's youth, when wise legislation was assumed to be a remedy for every public ill. Greg was interested in many philanthropic works.