When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom have survived to the present day, for they will have been supplanted by their successors, which were gradually rendered more perfect through natural selection.
Egituraren bat aztura partikular baterako zeharo hobeturik ikusten dugunean, esate baterako txori baten hegoak hegaz egiteko, kontuan izan beharko genuke lehen iragaitza mailak zituzten animaliak nekez biziraunen zutela egundaino, beren ondorengoek, hautespen naturalaren bidez gero eta hobetuago bilakatuko zirenek, ordezkatuko zituztelako.