[2] Born in Scotland, Davidson attended the University of Glasgow, where he graduated CM MB[3] in 1881 and received his higher MD in 1887.

He served a medical internship in the Glasgow Western Infirmary and then went into private practice at Thornhill, Dumfriesshire.

He became an associate professor of dermatology at the University of Southern California, while simultaneously studying natural history and publishing papers on botany and entomology.

[2] After an extended stay in Clifton, Arizona, he published the first floristic account for that state and in 1923 collaborated with George L. Moxley to produce a flora of southern California.

[2]In 1897 he married Alice Jane Merritt (1859–1931), author of California Plants in Their Homes: A Botanical Reader for Children (1898).