I am not quite sure why you think I have underestimated the importance of Malthus to Darwin; I won't repeat my argument. I tend to agree with the so called conservatives on this question: Ernst Mayr, Michael Ruse, Gertrude Himmelfarb.
By the way, I don't what I wrote that gave the impression that I thought RA Fisher was writing in the 1830s.
I learned of Fisher's criticism in Elliot Sober's The Force of Selection.
But maybe there is a debt to Adam Smith. it could be argued that as Smith believed that the greatest wealth and population were made possible by the intensive division of labor (that is, the growth of population is induced by the growth in productivity, which is in turn faciliated by the growing numbers of people engaged in a more intensive division of labor), Darwin argued that only by species divergence, a natural analogue to the division of labor, are the auto destructive consequences of Malthusian competition overcome and the greatest amount of life possible supported in a given area of the surface of the earth (since similarity within a niche only increases competition as variation is destroyed and the fittest trait is driven to dominance, spoiling the advantage which had accrued to it, natural selection will favor mutants which can exploit a new niche as a result of a new heritable trait).
That is we could read The Origin of Species as a kind of Smithean Wealth of Nature, as indeed the distinguished historian of science Sylvan Schweber seems to do in his contribution to the Darwinian Heritage, ed. David Kohn. Schweber argues that both Smith and Darwin are united in that they understand the division of labor/diversification in terms of the maximalization of an abstract quanta (wealth of nations, life per unit of land).
One could also argue Darwin was influenced by Babbage, especially his apologia of gluts (though no one has to the best of my knowledge). Far from denying the possibility of gluts, Babbage suggested they could stimulate "ingenuity and new methods of manufacturing." He even argued that if no such new methods are discovered, the adjustment may "drive the smaller capitalist out of the market [but] no artificial restraint should interfere to prevent it." I know of no discussion of whether Darwin was influenced by Babbage's apologia of gluts. But if Babbage reasoned that gluts induce the progressive development of technology and the winnowing out of unfit firms/industries, Darwin found in nature not only freedom from artificial restraints on the competition to which overpopulation gives rise but also--and as a result of such competition--the development and diversification of species along with the elimination of unfit variants. It's almost as if Darwin's Origin can be read as a natural justification of Babbage's defense of gluts--proof of the beneficience of an absence of the kind of artificial restraints for which Sismondi was calling--the unpleasantries of overproduction and overpopulation vindicated!
Or as Darwin writes near the end of Origins: "Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows."
(Simsondi had married Darwin's wife Emma's favorite aunt, and Babbage was a frequent dinner guest.)
best, rakesh