Yeah, but Spencer's "theories" of social evolution were also developed before Spencer himself knew anything of Darwin's ideas about biological evolution. (Spencer would insinuate himself into the "Darwin Circle" but he was a late addition.) IOW "Social Darwinism" -- certainly the original Spencerian version thereof -- is logically independent of Darwinian evolution. The only real connection is the incorporation of Darwin's name.
Look, here's Spencer's Law of Universal Evolution. You tell me if you think it has anything to do with Darwin. Or anything to do with anything for that matter:
Evolution is an integration of matter and concomitant dissipation of motion; during which the matter passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity; and during which the retained motion undergoes a parallel transformation.
This prop-wash is typical of Spencer. The trumped-up, bloviating idiot had a "law" for everything. William James quoted a biting parody of Spencer's law of evolution by a mathematician named Kirkman:
Evolution is a change from a nohowish untalkaboutable all-alikeness to a somehowish and in general talk-aboutable not-all-alikeness by continual stickingtogetherations and somethingelseifications.
William James in his own words derided Spencer for his "dry school-master temperament ... his preference for cheap makeshifts in argument, his lack of education even in mechanical principles, and in general the vagueness of all his fundamental ideas, his whole system wooden, as if knocked together out of cracked hemlock boards." Charles Darwin made similar (if less unfriendly) comments about Spencer in his correspondence. (As I recall he complained that Spencers ideas were "purely deductive" and failed to usefully engage or organize facts.)
Thanks for a great tag-line.