The concept of homology enables one to celebrate the differences or the similarities between two structures. Whether one emphasizes the similarities between our forelimb and a bird's wing or the differences between them depends on what you are describing. Comparative anatomy—with its Aristotelian and Cuverian interest in relating structure to function—usually emphasizes the differences. Morphology—with its Platonic and Geoffroyan interest in the underlying unities of structure—usually focuses on the similarities.
As Joseph Needham noted, embryology has swung between these two poles during different stages of its morphogenesis. In the late 1800s, the morphological tradition prevailed, and the similarities between developmental stages in different organisms constituted some of the best evidence for classification (see Nyhart 1995; Bowler 1996). Thus, the discovery of the Nauplius stage of the barnacle showed that it was a modified crustacean, and the notochord-containing tadpole of the tunicate demonstrated its affinities with the chordates. However, after the 1920s, embryology was no longer a major support for evolutionary biology, and the comparative anatomy tradition came to predominate. Until the 1980s, embryology was extremely descriptive. Each organ was seen to develop differently from any other organ, and each species was seen to develop differently from any other species. Indeed, embryology was defined (by one of its practitioners, E. G. Conklin) as a "lawless science," because generalities could not be made from the observations of animal development. When embryology underwent its anagenic transition to become developmental biology, the similarities among organisms were again emphasized. The similarities were now posited on the molecular rather than the morphological or cell lineage level. The 1990s has seen a remarkable celebration of the similarity of molecular processes throughout the animal kingdom. Homologous genes abound (the Hox genes, fringe, tinman, and Pax6 being seen to specify the anterior-posterior axis, the limb, the heart, and the eye, respectively, of organisms as diverse as insects and flies). Even signalling pathways are seen as being homologous both within a developing organism and between organisms. Thus, the neural tube in vertebrates and insects are seen as being formed through the same interactions of the "same" proteins, even though one neural tube is dorsal and the other ventral. The Richardson et al. paper reminds us that despite these similarities, differences are also important, especially if one is thinking in terms of the relationships between development and evolution.
.....cracks me up.
