On the Origin of Species · ¶299

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 continue to exist to the present day, for they will have been supplanted by the very process of perfection through natural selection. Furthermore, we may conclude that transitional grades between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. Thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of perfection, so as to have…

Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
Passage preview. Read in context for the complete text, notes and references.