'Theory' promoted during 1889 by Dr.Charles Edouard Brown-Sequard. Dr.Brown-Sequard was a French-American neurophysiologist as well as an endrocrinologist and a physician. Brown-Sequard believed that injections of liquified animal testicles (usually those of dogs or guinea pigs would increase libido and sexual abilities in elderly men (19th century viagra!) The idea was very controversial and quickly disproven. However, research that resulted from the controversy led to the isolation of testosterone. It is probably also worth noting that Brown-Sequard was 72 in 1889.

Testicular extracts are still sold today.