Building on the work of Darwin and Mendel, the biologist William Bateson (1861–1926) was the first scientist to combine the study of variation, heredity and evolution, and to use the term 'genetics'. This book was first published in 1894 after many years of experimental and theoretical work - particularly in the embryology of the acorn worm genus Balanoglossus - which had been guided by the principle that embryonic developmental stages replay the evolutionary transitions of adult forms of an organism's ancestors. Bateson was the first to challenge this theory, which made him unpopular among the scientific establishment of the time, but he was proved right. Organising his material by anatomical sections, Bateson explores speciation, phylogeny and discontinuous and continuous variation among a wide range of species, including vertebrates, invertebrates and plants. This pioneering work offers great insight into how the study of genetics and inheritance itself evolved.