EVIDENCE FROM TAILS, LIMBS, 6- LUNGS OF ANIMALS. 113 



in structure favourable to the preservation of the animal and its race; such 

 favourable variations giving it an advantage in the " struggle for exist 

 ence. " This principle satisfactorily enough accounts for the modification 

 of limbs to suit the varying habits of life which from time to time were 

 assumed by Vertebrate animals, as the new races and groups sprang 

 into existence by the modification of the older and more primitive 

 stocks. And the presence of the varied scheme of the Vertebrate 

 life of to-day the active bird,, the crawling serpent, the lithe fish, 

 the fleet steed, the aerial bat, and even the erect ruler of the universe 

 himself in this view, appears but as a testimony to the operation of 

 a great law of nature, which decrees that the newer and stronger 

 shall possess the earth, whilst the weak and primitive are at the same 

 time prevented, and perhaps wisely, from cumbering the ground. 



The subject of the origin of limbs, however, still awaits our brief 

 study. At various periods in the history of comparative anatomy, 

 the original nature of limbs has formed a subject regarding which 

 very diverse opinions have been expressed. Owen long ago regarded 

 limbs as corresponding to processes or appendages of ribs ; Maclise 

 represented them a little later as modified ribs ; and other autho- 

 rities have propounded theories in which the limbs are,, regarded as 



FIG. 50. THE CERATODUS OR BARRAMWNJJA^ 

 A, skeleton showing a the ventral, and b pectoral fins ; B, the fish itself. 



corresponding to outgrowths from a peculiarly modified gill-arch, 

 the latter structure forming the supporting " girdle " of the limbs or, 

 in the case of the fore-limb, the " shoulder." Recent researches 

 into the development of the fins of fishes to which we naturally turn 

 for the most primitive form of limb extant appear to lead to the 

 declaration that there is no real difference in nature to be perceived 

 between the " paired " and " unpaired " fins (see Fig. 38). The paired 

 fins of the dog-fishes and sharks are known to arise as special develop- 



