The concepts of analogy and homology are probably easier to exemplify than to define. When different species are structurally compared, certain features can be described as either analogous or homologous. For example, flight requires certain rigid aeronautical principles of design, yet birds, bats, and insects have all conquered the air. The wings of all three types of animals derive from functions. In this case, the light organs of creatures can be said to be analogous. In contrast, features that arise from the same structures in the embryo but are used in different functions are said to be homologous. The pectoral fins of a fish, the wings of a bird and the forelimbs of a mammal are all homologous structures. They are genetically related in the sense that both the forelimb and the wing evolved from the fin.