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Show 182 DIFFICULTIES ON THEORY. CHAP. VI. . . d th t b' ·d might have existed which used their su.r mise a n fl. s l'k the logger-h ea d ed d uck .... . wings solely as appers, 1 e (M !.C rop t crus of Eyton)., as fins in the wate. r an.d front legs on the land, like the penguin ; as sails, ~Ike the t .. h . d functionally for no purpose, hke the os nc , an f h b. d . Ap t eryx. Yet the structure of each . o t eseh ' 1n· .s 1.s d £ 't under the conditions of hfe to w IC 1 It IS goo or 1, 1 b . . exposed, for each has to live by a strugg e ; . ut It 1s not necessarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the .grades of wing-structure here ~llude~ t~, which perhaps may all have resulted from disuse, Indicate the natural steps by which birds have acq uirecl their perfect power of flight ; but th~y. serve, at l~ast, to show what diversified means of transition are possible. "l Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live on the land, and seeing that we have flying birds and mammals, flying insects of the most diversified types, and formerly had flying ~eptiles, it is concei vab!e that flying-fish, which now glide far through the au, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. If this had been effected, who would have ever imagined that in an early transitional state they had been inhabitants of the open ocean, and had used their incipient organs of flight exclusively, as far as we know, to escape being devoured by other fish? 'Vhen 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 continuo to exist to the present day, for they will have been supplanted by the very process of perfection through natur~l selection. Furthermore, we may conclude that transi- .CHAP, VI. TRANSITIONAL HABITS. 183 tion~l gra~es b.etween structures fitted for very different ha~Its ?f hfe will rarely have been developed at an early penod 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 ~any subordinate forms, for taking prey of many kinds In many ways, on the land and in the water, until their organs of flight h~d come to a high stage of perfection, so as t~ have. given them a decided advantage over other animals In the battle for life. Hence .the chance of discovering species with transitional grades of structure in a fossil condition 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 of diversified and of changed habits in the individuals of the same species. Wh~n either case o?curs, it would be easy for natural selection to fit the animal, by some modification of its stru.cture, for it.s changed habits, or exclusively for one of Its several different habits. But it is difficult to tell, and immaterial for us, whether habits generally chan.ge ~st and structure afterwards ; or whether slight modrfications of structure lead to changed habits ; both probably often change almost simultaneously. Of cases of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances. Of diversified habits innumerable instances could be given : I have often w_atched a tyrant flycatcher (Saurophagus sulphuratus) 1n South America, hovering over one spot and then proceeding to another, like a kestrel and at other times sta~ding stationary on the margin ~f water, and then dashing like a kingfisher at a fish. In our own country the larger titmouse (Parus major) may be |
