> in his mature work he assumes the uniqueness of capitalism
> as a social system, and his "laws" of history apply only to
> capitalism.
"Internally related" means that the "essence" of one stage, e.g. "socialism", requires for its existence its relation to the preceding stage, e.g. "capitalism" in the case of "socialism".
As Marx understands them, these "stages" are "educational" "stages in the development of the human mind", i.e. stages in an historical process of "bildung" ending in individuals who are "educated persons" in Hegel's sense.
The "laws" of capitalist development (including "the absolute general law of capitalist accumulation") ultimately concern the "development of the human mind" as this is brought about within capitalism, a development that Marx, both in the "early" 1845 German Ideology passage and in the "mature" 1877 letter identifies with the development of "the productive forces of social labour" and "the integral development of every individual producer".
"Socialism", as the "appropriation" of "the productive forces of social labour", requires for its own existence both these "all- embracing" "productive forces of social labour" and individuals with the degree of "integral development" such "appropriation" requires (since "only individuals that are developing in an all-round fashion can appropriate them, i.e., can turn them into free manifestations of their lives").
So "capitalism" is "internally related", in the above sense, to "socialism".
It's only through its "development of the human mind" in these ways that "it begets its own negation with the inexorability which governs the metamorphoses of nature".
Ted