Darwin had debated whether to publish a theory of heredity for an extended period of time due to its highly speculative nature. He decided to include pangenesis in "Variation" after sending a 30 page manuscript to his close friend and supporter Thomas Huxley in May 1865, which was met by significant criticism from Huxley that made Darwin even more hesitant. However, Huxley eventually advised Darwin to publish, writing: "Somebody rummaging among your papers half a century hence will find Pangenesis & say 'See this wonderful anticipation of our modern Theories—and that stupid ass, Huxley, prevented his publishing them'" Darwin's initial version of pangenesis appeared in the first edition of "Variation" in 1868, and was later reworked for the publication of a second edition in 1875.