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Show 100 HYPOTHESIS OF THE DEVELOPMENT OF of full-grown animals, we find all others to be mere.ly advances from that tvpe, with the extensjon of endow. ments and modification of forms which are required i~ each pa1·ticular case; each form, also, retaining a strong affinity to that which precedes it, and tending to impress its own features on that whjch succeeds. This unity of structure, as it is called, becomes the more remarkaole, when we observe that the organs, while preserving a resemblance, are often put to different uses. For example: the ribs become, in the serpent, organs of locomotion, and the snout is extended, in the elephant, into a prehensilf' instrument. It is equally remarkable that analogous purposes are served in different animals by organs essentially different. Thus, the mammalia breathe by lungs; the fishes, by gills. These are not modifications of one organ, but distinct or. gans. In mammifers, the gills exist and act at an early stage of the freta! state, but afterward go back and appear no more; while the lungs are developed. In fishes, again, the gills only are fully developed; while the lung structure either makes no advance at all, or only appears in the rudim~ntary form of an air-bladder. So, also, the baleen of the whale and the teeth of the land mammalia are different organs. The whale, in embryo, shows the rudiments of teeth; but these, not being wanted, are not developed, and the baleen is brought forward instead. The land animals, we may also be sure, have the rudiments of baleen in their organization. In many instances, a particular structure is found advanced to a certain point in a particular set of animals (for instance, feet in the serpent tribe), although it is not there required in any degree; but the peculiarity, being carried a little farther forward, is perhaps useful in the next set of animals in the scale. Such are called rudimentary organs. With this class of phenomena are to be ranked the useless mammCE of the male human being, and the unrequired process of bone in the n1ale opossum, which is needed in the female for supporting her pouch. Such curious features are most conspicuous in animals which form links between Yarious classes. As formerly stated, the marsupials, standing at the bottom of the mammalia, show their affinity to the oviparous vertebrata, by the rudiments of two canals passing from near the anus lo thE external surfaces of the viBcera, which THE VEGETABLE AND ANIMAL KINGDOMS. 101 are full~ developed in fishes, being required by them for the respiratwn of aerated waters, but which are not needed by the at~osphere-breathing marsupials. We have also the peculiar form of the sternum and rib-bones of the lizards represented in the mammalia in certain white car· tilaginous lines traceable among their abdominal muscles. ['he struphionidre (birds of the ostrich type) form a link (,etween birds and mammalia, and in them we find the Nings ~mperfectly or not a.t all developed, a diaphragm lnd unnary sac (organs wanting in other birds,) and feathers approaching the nature of hair. Again, the ornithorynchus belongs to a class at the bottom of the mam~ alia, and approximating to birds, and in it behold the \;>Jll and the web-feet of that order! .F'o:f farther illustration, it is obvious that, various as may ~e the lengths ?f t.he upper part of the vertebral col umn In the mamrnaha, It always consists (Jf the same parts. The giraffe has in its tall neck the same number of bones with the pig, which scarcely appears to have a neck at alL* lVIan, again, has no tail; but the notion of a muchrid! culed philoso_pher of the last century is not altogether, as It happens, Without foundation, for the bones of a caudal extremity exist in an undeveloped state in the os coccygis of the human subject. The limbs of all the vertebra~ e animals are, in like manner, on one plan, however vanous they may appear. In the hind leg of a horse, for ~xample, th~ angle called the hock is the same part which 1n us forms the heel; and the horse, and all other quad· ruped~, with. almost the solitary exception of the bear, walk, In reahty, upon what answer to the toes of a human being. In thi~ .an~ many other 9uadrupeds the fore pai·t of the ext:em~hes Is shrunk up In a hoof, as the tail of a human being Is shrunk up in the bony mass at the bottom of the back. The bat, on the other hand, has these par~s lar_gely .developed. The membrane, commonly calle? Its Win~, 1s framed chiefly upon bones answering preCisely to those of the human hand· its extinct congener ~he pterodactyle, had th~ sa~e me~b~ane extended upo~ the fore-finger only, wh1ch In that animal was prolonged to an extraordinary extent. In the paddles of the. whale, and other animals of its order, we see the same bones as in the more highly developed extrernities of the land • Daubenton established the rule, that all t e viviparous quad rupeds have seven vertebrre in the neck. |