 Marshall's first position was a research assistant to James Cossar Ewart in Edinburgh, assisting Ewart's work on the now-discredited theory of telegony.He also began his research on reproduction, studying the reproductive cycle of sheep at Ewart's Penicuik farm; this work resulted in his first significant research paper in 1903.He subsequently studied the oestrus cycle of ferrets (with Edward Schäfer) and dogs (with William A. Jolly).