The laws of Coexistence;-the adaptation of structure to function; and to a certain extent the elucidation of natural affinities may be legitimately founded upon the examination of fully developed species;-But to obtain an insight into the laws of development,-the signification or bedeutung, of the parts of an animal body demands a patient examination of the successive stages of their development, in every group of Animals.

The Hunterian Lectures in Comparative Anatomy, May and June 1837 (ed. University of Chicago Press, 1992) - ISBN: 9780226641898