Complex sentence: In 1880 he chaired a lecture by Charles Bradlaugh, with whom he disagreed over the issue of birth control, and soon became associated with the branch of secular thought led by Charles Watts and his son C. A. Watts.
Split into simpler sentences: In 1880 he chaired a lecture by Charles Bradlaugh, with whom he disagreed over the issue of birth control. Ross became associated with the branch of secular thinking led by Charles Watts and his son C. A. Watts.