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