Homology in development and the development of the homology concept

SYNOPSIS. Homology is a central concept for Developmental Evolution. Here I argue that homology should be| explained within the reference processes of development and evolution; development because it is the proximate cause of morphological characters and evolution because it deals with organic transformations and stability. This was already recognized by Hans Spemann in 1915. In a seminal essay "A history and critique of the homology concept" Spemann analyzed the history and present problems of the homology concept. Here I will continue Spemann's project and analyze some of the 20th century contributions to homology. I will end with a few reflections about the connections between developmental processes and homology and conclude that developmental processes are inherent in (i) the assessment of homology, (ii) the explanation of homology, (iii) the origin of evolutionary innovations (incipient homologues), and (iv) can be considered homologous themselves.