FIG. 38. PERCH. 



dorsal fins ; c, caudal fin ; a, anal fin ; 

 p, pectoral, and v, ventral fin. 



. CHAPTERS ON EVOLUTION. 



Very material tasHion by the consideration that in other species of 

 Macaques the tail has actually become thoroughly abortive. It is 

 difficult or impossible to explain, save on the theory of gradual modi- 

 ^i fication affecting species in 



different ways and at dif- 

 ferent rates, why one species 

 of monkey should have a 

 fairly developed tail, whilst 

 in another and nearly related 

 species the tail has well-nigh 

 disappeared. This disserta- 

 tion on the tail as represented 

 in human existence, may pre- 

 face the brief dissertation on 

 the tails of fishes, the con- 

 sideration of which in its 

 own way teaches us a lesson in evolution equally plain with that 

 drawn from the confines of quadruped existence. 



The tails of fishes, as everyone knows, 

 are set vertically (Fig. 38, <r), so ' that 

 the flat surfaces of the tail-fin correspond 

 with the sides of the body. The fish in 

 this respect differs materially from the 

 whale (Fig. 39) or dolphin, in which the 

 tail is placed horizontally, or across the 

 body. When a review of the tails of fishes 

 is attempted, two very distinct forms of this 

 appendage are discerned. In most fishes 

 the tail may be described as symmetrical 



when unforked (Fig. 38), or as possessing its halves of equal size 

 when forked (Fig. 40). But in other fishes, and most notably in 



such fishes as the stur- 

 geons, sharks, and dog- 

 fishes, the upper half of 

 the tail is seen to be dis- 

 proportionately developed 

 when compared with the 

 lower half. In such a fish 

 as the fox-shark (Fig. 41) 

 or thresher both names, 

 indeed, being derived 

 from the peculiarity in 

 question the upper lobe 

 of the tail appears relatively enormous when compared with the lower 

 half. Such are the external appearances of fishes' tails; and from their 



FIG. 39. 

 HORIZONTAL TAIL OF WHALE. 



FIG. 40. FISH SHOWING AN EQUAL-LOBED TAIL. 



