William Bateson (1861–1926) is most remembered as the man who introduced Mendelian genetics into Britain, propounded heredity as the mechanism of evolutionary change, and coined the name ‘‘genetics’’ for this new field of scientific endeavor. Less well known is Bateson’s early professional life as an evolutionary morphologist/ evolutionary embryologist studying the wormlike marine organism, Balanoglossus. His aim was to uncover the origin of the vertebrates; was Balanoglossus ‘‘the last of the invertebrates or the first of the vertebrates’’ (Crowther, ’52: p 253). Bateson, however, switched from embryology to heredity. How Bateson came to feel betrayed by Balanoglossus and how and why he made the switch from what he termed ‘‘the embryological method’’ to genetics and the analysis of variation as the means to understand evolutionary change are the topics of this paper. It is a contribution to the history of the now flourishing field of evolutionary developmental biology (‘‘evo-devo’’), which is the current manifestation of a field with a long history seeking relationships between embryology and evolution (Bonner, ’82, 2000; Hall, ’92, ’98, 2004, 2005; Raff, ’96). The significance of Bateson’s scholarship for the relationship between embryology and evolution was captured by Peter Bowler in a Foreword to the reprinting of Bateson’s Materials for the Study of Variation: ‘‘Biologists who retain an interest in the developmental process and its relationship to evolution may find his [Bateson’s] work illuminating for its ability to identify topics and problems which have been set aside in the construction of the genetical theory of natural selection. The few who wonder if a full recognition of the role played by development will threaten the basic principles of Darwinian adaptationism can see how far one of the pioneers of genetics was willing to carry this line of argument’’ (Bowler, ’92: p xxiv–xxv).