Posina Venkata Rayudu observed:
MacLane (1950) Duality for Groups ams.org/journals/bull/1950-56-06 ... doesn't cite Samuel (1948) Universal Mappings ... ams.org/journals/bull/1948-54-06 with its Universal Mapping Property...
Even more strange is that Mac Lane had REVIEWED Pierre Samuel's paper (needs an AMS login): mathscinet.ams.org/mathscinet-getitem?mr=0025152 Samuel probably also wrote the draft appendix to Bourbaki's "Algèbre" but it never appeared. This might be it: sites.mathdoc.fr/archives-bourbaki/PDF/041_iecnr_050.pdf Daniel Kan's 1958 paper "Adjoint Functors" was heavily citied withint a few years. Neither Kan nor Ehresmann cited Samuel. Google Scholar lists citations of Samuel in 1950+, 52, 57, 59+, 6-, 65+, 66, 70 (where + means twice) I haven't looked at all these but MacLane finally makes the connection with adjunctions in "Categorical algebra", Bull AMS 71 (1965) 40-106 pubs.ams.org/journals/bull/1965-71-01 These comments are based on fairly brief use of Google Scholar and other websites. It would be great if the few survivors from that time could correct me. Paul Taylor http://www.paultaylor.eu/delivery/261020-categories-history