Two of Darwin's staunchest supporters in the field during a trip to the Rockies in 1877. Left (seated) British botanist Joseph Dalton Hooker, the director of Kew and one of Charles Darwin's close friends. Also shown Dr. Ferdinand Hayden famous for his 1871 survey of Yellowstone. Seated on the ground with plant press in hand is American, Asa Gray, whose series of positive review of Darwin's 'Origin of Species' in the Atlantic Monthly magazine helped secure his theories scientific acceptance from many in the United States. Both were regular correspondents of Darwin who believed his theory provided the best framework for their own empirical observations.