CHAPTERS ON EVOLUTION. 



very material fashion 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- 

 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- 

 s PERCH ^ ace tne brief dissertation on 



rf 1 </', dorsal fins ; c, caudal fin ; a, anal fin ; the tails of fishes, the COn- 



/, pectoral, and v, ventral fin. 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, c\ 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. 



