Probably - in one way it is true (see for example Robert Brenner "Agrarian Class Structure and Economic Development in Pre Industrial Europe" _Past and Present_ 97, 1982.), but some argue (Blaut & Co, inter alia) that the plunder of the Third World was the decisive factor which implies that the ability to plunder was something unique and separate from the agrarian relations. I do not think that the latter argument has much weight in terms of explaining the main impetus of capitalist development - yeah, there was plunder, but to cross the pond and heist the boatloads of gold you need to be already fairly advanced technologically and organizationally, no? But it raises an interesting question whether capitalism was a "natural" outgrowth of the feudal agrarian relations - a stagiest approach to history may suggest - or there was some unique value added that sparked its development.
Brenner argues the latter but with a twist - it was a unique class relations peculiar to England that led to the emergence of capitalism there rather than elsewhere - i.e. a historical accident that set a new path, and explanation that I like a lot because it is not deterministic and does not fall into the "great actors" genre.
But I also think that the 'agrarian roots' approach is more true about the working class than capitalists. The emergence of the entrepreneuring class in a particular form may be due in part to historical accident, but "rural idiocy" or more precisely pre-modern agriculture invariably produces a large pool of unskilled and "redundant" (relative to available resources) people available as an "input" for industrial economies either internally (e.g. England) or overseas (e.g. Ireland, or Eastern Europe prior to central planning). The particular form of industrial economy did not create that "input" - it merely found a use for it. The conditions of that use (more or less exploitative) and the future transformation of the class being used that way depends, of course, on the specifics of the industrial organization (cf. slavery in the US, wage labor in England, and wage labor in socialist countries) - but the class itself and its initial properties were a "given," a product of a different system of production.
I think an important lesson of it is that capitalism is not a coherent system but rather a series of fortuitous "Midas touches" with path dependencies - there was an opportunity, someone took advantage of it and used it to further advantages thus creating path dependencies. It is a 'system" without any coherent logic than can be more properly thought of as "muddling through." That means that there will be no final crisis that does the beast in only a series of downturns that will spur new adaptations. One capitalist may lose, but another one will gain, and the "system" will continue to replicate itself. The best the left can for is to reform and modify it to its values as much as possible.
Wojtek