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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 given them a decided advantagepver 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. When either case occurs, it would be easy for natural selection to fit the animal, by some modification of its structure, for its changed habits, or exclusively for one of its several different habits. But it is difficult
~ to tell, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications 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 watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot .and then proceeding to another, like a kestrel, and at other times‘ standing stationary on the margin of water, and then dashing like a king-fisher at a fish. In our own country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper; it often, like a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the seeds of the yew on a branch, ‘and thus breaking them like a nuthatch. In North America the black hear was seen by Hearne swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water.
170 DIFFICULTIES ON THEORY
As we sometimes see individuals of a species following habits widely different from those of their own species and of the other species of the same genus, we might expect, on my theory, that such individuals would occasionally have given rise to new species, having anomalous habits, and with their structure either slightly or considerably modified from that of their proper type. And such instances do occur in nature. Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing insects in the chinks of the bark ‘2 Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing. On the plains of La Plata, where not a tree grows, there is a woodpecker (Colaptes campestris) which has two toes before and two behind, a long pointed tongue, stiff pointed tail feathers, but not so stiff as in the typical woodpeckers (yet I have seen it use its tail when alighting vertically on a post), and a straight strong beak. The beak, however, is not so straight or strong as in the typical woodpeckers; but it is strong enough to bore into wood; and I may mention, as another illustration of the varied habits of the tribe, that a Mexican Colaptes has been described by De Saussure as boring holes into hard wood in order to lay up a store of acorns for its future consumption! Hence the Colaptes of La Plata in all the essential parts of its organization is a woodpecker, and until recently was classed in the same typical genus. Even such trifling characters as its colouring, the harsh tone of its voice, and undulatory flight, all told me plainly of its close blood-relationship to our common species; yet it is a woodpecker, as I can assert not only from my own observation, but from that of the accurate Azara, which never climbs a tree!
Petrels are the most aerial and oceanic of birds, but in the quiet Sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when unwillingly it takes flight, would be mistaken by any one for an auk or a grebe; nevertheless, it is essentially a petrel, but with many parts of its organisation profoundly modified in relation to its new habits of life; whereas the woodpecker of La Plata has its structure only slightly modified. In the case of the water-ouzel, the acutest observer by examining its dead body would never have suspected its sub-aquatic habits; yet this anomalous member of the terrestrial thrush family wholly subsists by diving,—grasping the stones with its feet, ' and using its wings under water.
He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not at all in agreement. What can be plainer than that the webbed feet of ducks and geese are formed for swimming? yet there are upland geese with webbed feet which rarely or never go near the water; and no one except Audubon has seen the frigate-bird, which has all its four toes webbed, alight on. the surface of the sea. On the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by membrane. What seems plainer than that the long toes of grallatores are formed for walking over swamps and floating plants ‘2 -—yet the water-hen is nearly as aquatic as the coot; and the landrail nearly as terrestrial as the quail or partridge. In such cases, and many others could be given, habits have changed without a corresponding change of structure. The webbed feet of the upland goose may be said to have become rudimentary in function, though not in structure. In the frigate-bird, the deeply— scooped membrane between the toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one of another type; but this seems to me only restating the fact in dignified language. He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may be from its own place. Hence it will “cause him no surprise that there should be geese and frigate-birds with webbed feet, living on the dry land or most rarely alighting on the water; that there should be long-toed corncrakes living in meadows instead of in swamps ; that there should be woodpeckers where not a tree grows ; that there should be diving thrushes, and petrels with the habits of auks.
172 DIFFICULTIES ON THEORY
Organs of extreme perfection and complication—To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. \Vhen it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false ; but the old saying of Voa: populi, voa: Dei, as every philos0pher knows, can never be trusted in science. Reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the varia— tions he inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal'under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound.
In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same 'group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye (though in the fish Amphioxus, the eye is in an extremely simple condition without a lens), and from fossil species we can learn nothing on this head. In this great class we should probably have to descend far beneath the lowest known fossiliferous stratum to discover the earlier stages, by which the eye has been perfected. .
In the great kingdom of the Articulata, we can start from an optic nerve, simply coated with pigment, which sometimes forms a sort of pupil, but is destitute of a lens or any other optical mechanism. From this rudimentary eye, which can distinguish light from darkness, but nothing else, there is an advance towards perfection along two lines of structure, which Miiller thought were fundamentally different; namely,—firstly, stemmata, or the so-called “ simple eyes,” which have a lens and cornea ; and secondly, “compound eyes,” which seem to act mainly by excluding all the rays from each point of the object viewed, except the pencil that comes in a line perpendicular to the convex retina. In compound eyes, besides endless differences in the form, proportion, number, and position of the transparent cones coated by pigment, and which act by seclusion, we have additions of a more or less perfect concentrating apparatus :‘ thus in the eye of the Meloe the facets of the cornea are “ slightly convex both externally and internally—that is, lensshaped.” In many crustaceans there are two corneas— the external smooth, and the internal divided into facets— within the substance of which, as Milne Edwards says, “ renflemens lenticulaires paraissent sé’tre développés ; ” and sometimes these lenses can be detached in a layer distinct from the cornea. The transparent cones coated with pigment, which were supposed by Miiller to act solely by excluding divergent pencils of light, usually adhere to the cornea, but not rarely they are separate from it, and have their free ends convex ; and in this case they must act as converging lenses. Altogether so diversified is the structure of the compound eyes, that Miiller makes three main classes, with no less than seven sub-divisions of structure; he makes a fourth main class, namely, “ aggregates” of stemmata, and he adds that “this is the transition-form between the mosaic-like compound eyes unprovided with a concentrating apparatus, and organs of vision with such an apparatus.”
With these facts, here too briefly and imperfectly given, which show how much graduated diversity there is in the eyes of our existing crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve