Page:The Scientific Monthly vol. 3.djvu/611

 �ORIGIN AND EVOLUTION OF LIFE 605

poda). The manner of this transformation has been one of the great- est problems in the solution of the origin of animal form since the earliest researches of Carl Gegenbaur of Heidelberg, who sought to derive the lateral fins from modification of the cartilaginous rods sup- porting the gills, through a profound change of function. While pa- leontology has disproved Gegenbaur's hypothesis that the limbs of the higher vertebrates, including those of man, are derived from the gill arches of fishes, it has helped to demonstrate the truth of Reichert's hj'pothesis that the bony chain of the middle ear of man has been derived through change of function from a portion of a modified gill arch {mandibular cartilage) of the fish.

The cycle of shark evolution in course of geologic time embrace; a majority of the swift-moving, predaceous types, which branch into the sinuous, elongate body of the frilled shark (Chlamydoselache) and into forms with broadly depressed bodies such as the bottom-living skates and rays. Under the law of adap- tive radiation the sharks seek every possible habitat zone in the search for food. The nearest approach to the eel-shaped type among the sharks are certain forms discovered in Carboniferous time. By Upper

Devonian time the fishes in general

, ■,, 1 T. 1 - 1 11 J.1 Pir.. 1.1. IUppbb.) Cladoaclache, thb

had already radiated into all the th'h of tfb rniyiTiTH devomin

great existing groups. The primi- rii*bk or Ohio with Paisrd and

■ ' 1 A II 1- ^ r\, ^^El>[AN Fins Photidkd with Rod-likb

tive armored Arthrodires and Ostra- cawii^o.nocs Supi-oirrs. from whioh

coderms were nearing extinction, fipe by lusion the limbs of au tu*

The shftrka wprp still in the earlv '^" '""^ 'er'^brates Save beeo da- me snarKS were siin m ine eariy ^„^ ^^^i f,^ ,^j^^ Husaakof and

lappet-fin stage of evolution above Horter from apvclmma Id the American

dcsctibea,. common characteristic ""."S^,"', "'V","' "'"S r,>-, or

of the family being that they never ciadotrinche Rhowino thb cahtilaoin-

evolved a bony armature. The °"^ ^"^ i«'"'^* within the fin

^ (Black). Aftar Dean,

scaled armature of the first true

Ganoid fishes {Osteolepis,Ckeirolepis) makes its first appearance. These armored knights of the sea are related to simpler forms which gave rise to the rich stock of sturgeons, garpikes, bow-fins, and true bony fishes (Teleosts) which now dominate all other fish groups both in the fresh- waters and the seas. Close to this stock are the first lung fishes (Dipnoi), represented by Dtplerus; also the "lobe-" or "fringe-finned" Ganoids from which the first land vertebrates were derived. From a single localitj', in the Old Red Sandstone of Scotland, Traquair has

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