There has been a literature of genetics ever since philosophers and scientists began considering the mechanisms of heredity by which physical traits passed from generation to generation. Empirical observation gave rise to various theories about how that transmission occurred. Generally, these ideas were bound up with theories of human reproduction. The ancient Greek philosopher Aristotle believed that semen was responsible for passing on traits, while the Greek physician of that same classical era, Hippocrates, developed a theory of pangenesis in which the material enabling heredity was collected from throughout the body. Amr ibn Bahr Al-Jahiz, ninth-century North African philosopher and zoologist, considered species’ struggles to survive in their environments. Enlightenment considerations of heredity still reﬂected these notions. The
observation and classiﬁcation of varieties of organic beings raised questions about how species maintained consistency from generation to generation and how changes might be introduced as part of a more comprehensive set of questions about evolution. In the late eighteenth century, Erasmus Darwin’s Zoönomia (1794-96) advanced the ideas that mammals derived from a single source or “ﬁlament” and that they acquired and passed on new traits developed in response to their environments. Jean-Baptiste Lamarck elaborated these ideas in Philosophie zoologique (1809), asserting that individuals develop new, useful traits, lose useless traits, and pass these alterations on to their progeny. In his Variation of Animals and Plants Under Domestication, published in 1868, nine years after On the Origin of Species (1859), Charles Darwin set out a mechanism of heredity in which an individual’s “pangenes,” circulating throughout the organism, gather traits and migrate to the reproductive cells. That Darwin’s theory of evolution needed some mechanism for the transmission of traits sparked greater interest in issues of heredity, producing some opposition to Lamarck’s ideas, especially on the part of August Weismann, a German evolutionary biologist. Weismann disagreed with Lamarckism and pangenesis, positing instead that germ cells were unaﬀected by the environment and, thus, that an individual’s acquired traits were not passed to the next generation.