I think Darwin's debt to Malthus is much exaggerated. However, it is true that the Malthusian overpopulation scenario of acute, immediate human struggle on the basis of a competitive exclusion principle over a limited food supply did help Darwin discover that survivorship is non fortuituously linked to intraspecific heritable differences. Darwin's possible debt to Malthus is no recent discovery; August Weismann had suggested it before the publication of Darwin's notebooks in which Darwin does announce that Malthus finally gave him a theory to work by. But note first that logically the theory of natural selection does not depend on Malthusian populaton foundations. Competition need not be a matter of differential deaths; it could result from differential fecundity based on comparative efficiency in the use and partition of resources, factors such as better ability to find mates, to enjoy longer reproductive years, to have more offspring. That was pointed out by RA Fisher. I think in 1837 Darwin had already entertained the possibility of "benign replacement of one form by another without explicit struggle"! Moreover, as a polemic against Godwin and Condorcet, Malthus' Essay on Population was more interested to demonstrate how overpopulation rendered change impossible; Darwin's vision of species divergence and incessant continuous change is at odds with the parson's conservative vision. For Darwin death is a creative force, not a stabilizing one as with Malthus. Perhaps Darwin thought that natural selection would take too long to work if operated solely through differential fecundity--that is "benign replacement".
Perhaps it was easier to imagine descent with modification if we we keep foremost in our minds the active and relentless operation of the Malthusian checks of mass starvation deaths of the greater number of descendants, disease and famine. And so Darwin urged his readers (along with brilliant use of the metaphor of animal breeding, though Wallace urged him to dispath that metaphor)
Darwin's theory was challenged because the ascertained age of the earth was too young to have allowed descent with modification. My guess is that Darwin thought the invocation of the Malthusian competitive exclusion principle made it more believable that natural selection could have effected observable modifications in descent in the little time the earth was thought to have possibly supported life. Lord Kelvin had laid down the challenge to Darwin that earth was simply not old enough for natural selection to have done so much work. But this is only my guess, I don't know if it washes, I haven't seen it argued anywhere else.
I think economists get pretty obnoxiously puffed up contemplating the scientific revolutionary's possible debt to one of their own. But as I have suggested, the theory of natural selction does not depend logically on Malthus' overpopulation scenario, however realistic or paradigmatic that may be in the natural world; Darwin predicts "wedging" and incessant change at odds with Malthus' static world view; and Darwin's theory is much more fundamentally based on the analogy to artificial selection/animal breeding to comprehend natural selection as a force and for awareness that intra-specific variety as the material of selection (much is made of Malthus' emphasis on intra-specific competition as crucial to Darwin).
best, rakesh
ps unrelated note: what a period of scientific creativity, the mid-19th century: Darwin, Mendel, Maxwell and Marx!