Dave- What you may or may not be looking for is Marshall's inaugural address entitled: "The Present Position of Economics" (1885). You can find it reprinted in Memorials of Alfred Marshall edited by Pigou. Marshall discusses that economics needs more students that have "the power of keeping the head cool and clear in tracing and analysing the combined action of many combined causes. Exceptional genius being left out of account, this power is rarely found save among those who have gone through a severe course of work in the more advanced sciences....And many of those who are fitted for the highest and hardest economic work are not attracted by the metaphysical studies that lie at the threshold of that tripos (Moral Science) ." (p. 171) Neil Niman