Re-watching Un Chien andalou yesterday (in the Dalí & Film exhibition at Tate Modern), one sequence struck me as possibly relevant to where Morris got the idea from. At one point, Buñuel has a sequence of Pierre Batcheff fondling Simone Mareuil’s breasts, intercutting between shots of him doing this through her dress and with her naked. Then there’s a shot of his hands fondling her naked backside, her cheeks in place of her breasts.
Since Morris has been a dedicated Surrealist painter since the Forties and knew Dalí, it seems likely that he saw the film - and took it seriously - long before he ever wrote The Naked Ape.
Speculation, of course, and largely irrelevant to the plausibilty or otherwise of his hypothesis.