Google has suggested it may have been Charles Darwin who said this; of Mathemeticians, but in fact it might go back to someone named Charles Bowen.

http://en.wikiquote.org/wiki/Charles_Darwin

http://en.wikipedia.org/wiki/Charles_Bowen,_Baron_Bowen

"A mathematician is a blind man in a dark room looking for a black cat which isn't there." This is attributed, with an expression of doubt as to its correctness, in Mathematics, Our Great Heritage : Essays on the Nature and Cultural Significance of Mathematics (1948) by William Leonard Schaaf, p. 163; also attributed in Pie in the Sky : Counting, Thinking and Being (1992). There are a number of similar expressions to this with various attributions, but the earliest published variants seem to be quotations of Lord Bowen:

"When I hear of an 'equity' in a case like this, I am reminded of a blind man in a dark room — looking for a black hat — which isn't there." Lord Bowen, as quoted in in Pie Powder", Being Dust from the Law Courts: Collected and Recollected on the Western Circuit, by a Circuit Tramp (1911) by John Alderson Foote; this seems to be the earliest account of any similar expression. It is mentioned by the author that this expression has become misquoted as a "black cat" rather than "black hat."