It is often claimed that the notion of natural transformations existed in mathematical vocabulary long before it had a definition (see, for example, Peter Freyd, **Abelian Categories**, p. 2). Eilenberg and Maclane in: *General theory of natural transformations*, Trans. AMS, 58 (1945) p.p.: 231-294, discovered the fact that this notion could be mathematically defined. 

However, Ralf Kromer casts doubt on the above claim due to lack of evidence (see: Ralph Kromer **Tool and Object: a history and philosophy of category theory**, p. 70).

My question is, can you supply an evidence of use of the notion of natural transformations prior to the 1945 paper of Eilenberg and Maclane cited above?