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Show :204 THE DESCENT OF MAN. PART I. Nevertheless it is certain that groups of animals have Bxisted, or do now exist, which serve to connect more or less closely the several great vertebrate classes. We have seen that the Ornithorhynchus graduates towards 1·eptiles; and Prof. Huxley has made the remarkable discovery, confirmed by Mr. Cope and others, that the ·old Dinosaurians are intermediate in many important respects between certain reptiles and certain birds-the latter consisting of the ostrich-tribe (itself evidently a widely-diffused remnant of a larger group) and of the Archeopteryx, that strange Secondary bird having a long tail like that of the lizard. Again, according to Prof. Owen,20 the Ichthyosaurians-great sea-lizards furnished with paddles-present many affinities with fishes, <>r rather, according to Huxley, with amphibians. This latter class (including in its highest division frogs and .toads) is plainly allied to the Ganoid fishes. These latter fishes swarmed during the earlier geological periods, and were constructed on what is called a highly generalised type, that is they presented diversified affinities with other groups of organisms. The amphibians .and fishes are also so closely united by the Lepidosiren, that naturalists long disputed in which of these two dasses it ought to be placed. The Lepidosiren and some few Ganoid fishes have been preserved from utter .extinction by inhabiting our rivers, which are harbours of refuge, bearing the same relation to the great waters of the ocean that islands bear to continents. Lastly, one single member of the immense and diver ·~ified c.la.ss of fishes, namely the lancelet or amphioxus, 1s so different from all other fishes, that Hackel maintains that it ought to form a distinct class in the vertebrate kingdom. This fish is remarkable for its 20 'Palreontology,' 1860, p. 199. CHAP. VI. AFFINITIES AND GENEALOGY. 205- neg.ative characters; it can hardly be said to possess a bram, vertebral column, or heart, &c.; so that it was classed by the older naturalists amongst the worms. Many years ago Prof. Goodsir perceived that the lancelet presented some affinities with the Ascidians which are invertebrate, hermaphrodite, marine crea~ tures per~anen~ly attached to a support. They hardly appear hke ammals, and consist of a simple touo-h leathery sack, with two. small projecting ;rifices: Thei belong to the Molluscmda of Huxley-a lower division of the great kingdom of the Mollu~ca; but thev have recently been placed by some naturalists amongst the Vermes or worms. Their larvre somewhat resemble tadpoles in shape,21 and have the power of swimminO' freely about. Some observations lately made by M. Kowalev~ky,22 since confirmed by Prof. Kuppfer, will form a discovery of extraordinary interest, if still further extended, as I hear from M. Kowalevsky in Naples he has now effected. The discovery is that the larvro of Ascidians are related to the V ert~brata, in their manner of development, in the relative position of the nervous: system, and i~ possessing a structure closely like the ?horda dorsahs of vertebrate animals. It thus appears, if we may rely on embryology, which has always proved the safest guide in classification, that we have at last gained a clue to the source whence the Vertebrata haye ~ 1 I had the satisfaction of seeing, at the Falkland Islands, in April~ 183~, and therefore some years before any other naturalist, the locomottv~ larvre.of. a compound Ascidian, closely allied to, but apparently genencally dtstmct from, Synoicum. The tail was about five times as long as. the ~b~ong het~d, and terminated in a very fine filament. It ":us plainly dtvtded, a~ ~ketched. by me under a simple microscope, by transverse opaque partttwns, whwh I presume represent the great cells figured b~ Kowalevsky. At an early stage of development the tail was closely cotled round the head of the larva. 22 'Memoires de l'Acad. des Sciences de St. Petersbourg,' tom. x. No. 15, 1866. |