Herbert Spencer Jennings (1868-1947) was an American zoologist, geneticist, and eugenicist. His research helped demonstrate the link between physical and chemical stimulation and automatic responses in lower orders of animals.

Jennings was born in 1868 in Tonica, Illinois. Jennings began his scientific education at the University of Michigan where he obtained his Bachelor of Science in 1893. He continued at Harvard from 1895- 1897 receiving his M.A. and Ph.D. From 1898 to 1906 he taught at Texas A & M, the University of Michigan, Harvard, Montana State College, Dartmouth and the University of Pennsylvania. In 1906 he came to John Hopkins University as Associate Professor of Zoology and in 1910 he made Henry Walters Professor of Zoology and director of the zoology laboratory of the University. He remained at Johns Hopkins University until his retirement in 1938.

His chief research interests were protozoology and genetics, and he was the author of 10 major works and numerous articles and lectures. Among his books which attracted international attention in the scientific world were: Behavior of Lower Organisms, Life and Death, Heredity and Evolution in Unicellular Organisms, Prometheus--or Biology and Advancement of Man, The Biological Basis of Human Nature, Genetics of the Protozoa, and The Universe and Life.

Jennings was an Associate Editor of The Journal of Experimental Biology, The Biological Bulletin, and Human Biology, and he was a trustee of the Marine Biological Laboratory at Woods Hole, Mass. He held the Leidy Award, Academy of National Science, Philadelphia; Walker Prize, Boston Society of Natural History; and was a member of many scientific organizations. He served as President of the Society of Naturalists in 1911 and of the Society of Zoologists in 1909.

After his retirement from The Johns Hopkins University in 1938, Jennings was stricken by illness in 1946, and he died on April 14, 1947 at Santa Monica, California.