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The central figure shows the skeleton with the wings (or patagium) outstretched.

The upper figure shows the favourite attitude of rest, with the wings folded up.

The lower figure shows the mode of progression along a branch, by means of the claws on the thumbs.

[N.B.-In this figure, the left-hand side represents the top, the righthand side the bottom of the figure.]

stages be useless for the purpose which it ultimately fulfils.

This very important objection applies to a great number of cases, of which the origin of the wing is a typical one.

2. The Origin of the Lung is another example of the difficulty we are considering. The lung develops as a small saccular outgrowth from the throat, and it is quite unintelligible that a slight depression at the back of the mouth should have been preserved because it was useful for breathing air directly.

The explanations in these cases are good instances of many which are afforded by an important theory.

THE THEORY OF CHANGE OF FUNCTION. This theory was suggested by Darwin, and afterwards developed more fully by Dohrn, as affording a possible solution of difficulties such as those we are considering. The principle is, that an organ may lose its original function, and yet persist because it is useful for another purpose-i.e., that an organ may be used for two or more different purposes, one predominating at one time, another at another time; and, further, that structural modifications may ensue fitting the organ better for its adopted function.

This theory offers an explanation of the first commencement of the lung, which is shown to have arisen from the swimming bladder of fishes through change of function, and a series of forms is known to exist connecting the air bladder of fishes with the lung of the higher Vertebrates, which is undoubtedly the same organ.

The swimming bladder of most fish-the sturgeon for example-is a closed sac lying beneath the vertebral column, and is used for the purpose of flotation, to keep the back uppermost. In many

F

E

D

C

B

A

FIG. 29.

Diagram showing Evolution of Lung from Swim-bladder of Fish.

A, Sturgeon; B, Ceratodus; C, Menobranchus; D, Amphiuma;
E, Newt; F, Frog.

fish it acquires a connection with some part of the alimentary canal, and then becomes an accessory breathing organ.

The mud-fish, Ceratodus, of Queensland and Protopterus of Africa, inhabit rivers which during the dry seasons are apt to become dried up. These animals lie buried in the mud for months, and can live for a long time out of water, owing to the fact that the swimming bladder is used as a lung, a slight change in the circulation causing aërated blood to be returned from it to the heart.

In Menobranchus, found in North America, both lungs and gills are present throughout life, and it is equally at home in water and on land. The lung is better developed than in the Protopterus and Ceratodus.

In Amphiuma, found in North American swamps, the lungs are still more perfect. The gills are lost but the gill-slits remain.

From this we reach the condition met with in the newt and frog, which possess gills in the tadpole stage, but lose them in the adult or lung-breathing state. (See Fig. 29.)

We have thus a series of animals, all now living, showing the actual transition from the swimming bladder to the lung, and from the gill-breathing to the lung-breathing condition; and we further see that the frog actually repeats this history in its own development.

The explanation of the first commencement of the bat's wing is more difficult, and there is still some uncertainty about it. Let us consider another group of Mammals, the SQUIRRELS, in which the finest gradation is known from animals with their tails slightly flattened, the hind parts of their bodies wide, and the skin of the flanks full, to the "flying squirrels," in which the limbs and even the base of the tail are united by a broad expanse of skin; this fold of skin, acting like a parachute, enables them to glide through the air for a great distance from tree to tree, and so escape their enemies. Here each step is useful, and similar modifications are met with in other animals.

In Galeopithecus, the flying lemur of Borneo, the skin fold extends from the neck to the hand, thence to the foot and from this to the tail, and includes the limbs with the elongated fingers. From this Darwin suggested that the bat's wing could be derived by elongation of the fingers.

Further illustrations of the utility of imperfect

The

wings to arboreal animals or fish are found. flying frog of Borneo is a tree-frog with very long and fully webbed toes, which enable it to take long leaps in the air. The flying lizard (Draco volans) has the skin of the flanks supported by ribs. In both these cases there is no true power of flight, the action being that of a parachute. Of flying fish there are two chief groups, Dactylopterus (the gurnard), and Exocoetus (the flying herring). In both animals the pectoral fins are largely developed, and in the gurnard are almost certainly moved like wings.

There is no difficulty in understanding these cases as being acquired by Natural Selection, and the bat's wing may be not such a serious difficulty after all.

Other Examples of Change of Function.-A good instance of change of function is that of the Hyomandibular gill-cleft. The presence of gill-slits in

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