"The web of mutual obligations between generations is essential for a civilized society."
This passage cannot be found in Thomas Jefferson's writings. It is most likely a corruption of author Joseph Ellis's summary of a letter that James Madison wrote to Thomas Jefferson. This was in response to Jefferson's "The Earth Belongs to the Living" tract. Ellis summarized Madison's critique of Jefferson's paper, in part, as follows: "...There is instead a seamless web of obligatory connections between past and present generations. These connections are not only unavoidable but absolutely essential for the continuation of civilized society."
In Madison's actual letter, the concept of the quotation mentioned above is even less concise. Probably the passage which most efficiently captures the idea is as follows: "There seems then to be some foundation in the nature of things; in the relation which one generation bears to another, for the descent of obligations from one to another. Equity may require it. Mutual good may be promoted by it."