EVIDENCE FROM TAILS, LIMBS, &> LUNGS OF ANIMALS. 103 



FIG. 41. THRESHER -OR FOX-SHARK. 



aspect when casually regarded we might seem fully justified in saying 

 that but two kinds of tails were developed in the fish class, namely, 

 equal and unequal tails. We must, however, inquire as to the verdict 

 which comparative ana- 

 tomy, with its deeper 

 research into the struc- 

 ture and composition 

 of parts, has to pro- 

 nounce on the like- 

 nesses or differences 

 which the superficial 

 view discovers. The 

 result of such an inquiry shows us that the tail of a fish, in Othello's 

 words, may be said to 



Beguile 

 The thing I am by seeming otherwise ; 



since we shall find that the tail of equal shape and conformation is 

 certainly not what it seems; and that, moreover, it possesses a singular 

 relationship to its unequal neighbour. When the bony framework 

 consisting of the end of the spine, which supports the tail, is duly 

 examined in certain fishes, such as the Polyptemis of African rivers, 

 the spine is seen to terminate in such a fashion that the rays of the 

 tail- fin are divided into two equal portions. In this case the tail is 

 both apparently and really symmetrical. But such a state of matters 

 is comparatively rare. The salmon, as 

 every one knows, has to all intents and 

 appearances a tail which is perfectly 

 symmetrical and equal. Yet when we 

 inspect the skeleton of the salmon's tail 

 (Fig. 42), we find a very obvious want 

 of symmetry. The extremity of the 

 tail is bent upwards, as depicted in the 

 illustration, so as to give a greater pre- 

 ponderance of spine (s s) to the upper FIG 



half, and the Symmetry Of the tail is SKELETON ^SALMON'S TAIL. 



preserved simply through the lower fin rays being more numerous and 

 longer than the upper ones. In those fishes (Fig. 41), on the other 

 hand, in which the tail is of unequal conformation, even in external 

 appearance, the upper half attains its greater development from the 

 spine extending boldly upwards, and from the inferior and rudi- 

 mentary development of the rays and elements of the lower half of 

 the tail. Thus summing up the knowledge regarding fishes 7 tails 

 which comparative anatomy supplies, we find that only a few fishes 

 possess really symmetrical tails that is, tails in which the spine 

 terminates in the middle line, and in which the fin-rays are given off 



