on pterosaurs, Dragons of the Air (1901), found that the development of birds and pterosaurs paralleled each other. His belief that they had a common origin has been proved, for both are archosaurs, just not as close as he thought. He upset Richard Owen's characterization of the pterosaurs as cold-blooded, sluggish gliders, and recognized them as warm-blooded active fliers.

He was elected a Fellow of the Royal Society