Page - 187 - in The Origin of Species

TRANSITIONS OF ORGANIC BEINGS 187 'fectly winged animals. If this had been cfTcctcd, who would have ever imagined that in an early transitional state they had been the inhabitants of the open ocean, and had used their incipient organs of flight exclusively, as far aswe know, to escape being devoured by other fish? When we see any structure highly perfected for any par- ticular 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 selec- tion. Furthermore, we may conclude that transitional states between structures fitted for very different habits of life will rarely have been developed at an early period in great num- bers andundermany 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 state of perfection, so as to have given them a decided advantage over other ani- mals in the battle for life. Hence the chance of discovering species with transitional grades of structure in a fossil con- dition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures. I will now give two or three instances both of diversified and of changed habits in the individuals of the same species. In either case it would be easy for natural selection to adapt the structure of the animal to its changed habits, or exclu- sively to one of its several habits. It is, however, difficult to decide, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits ; both prob- ably often occurring almost simultaneously. Of cases of changed habits it will sufiice merely to allude to that of the many British insects which now feed on exotic plants, or ex- clusively on artificial substances. Of diversified habits innu- merable instances could be given : I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South Amer-