790 Comparative Morphology <>i' Chordates 



cental." There cannot be any genetic connection between the "pla- 

 centa" of a shark and that of a lizard or mammal, nor between the 

 "placenta" of a lizard and that of a mammal. Extreme elongation of 

 body, along with reduction of the locomotor appendages, is a line of 

 specialization which has been followed by several groups quite in- 

 dependently of one another. Among teleost fishes it has produced the 

 common eels and the large conger eel. Amphiuma and Siren (Fig. 349) 

 are amphibian "eels." Snakes are the reptilian equivalent of eels. It is 

 possible that the resemblances between legless lizards (amphisbae- 

 nians) and snakes are a case of parallel, instead of convergent, evolu- 

 tion (see p. 366). Such long-bodied, short-legged mammals as the mink 

 and ermine (Figs. 558, 603) are somewhat snakelike in external form. 

 The absence of locomotor appendages in the cyclostome eels is probably 

 not the result of reduction. The cyclostome line probably diverged 

 from the primitive chordate stock before paired locomotor fins had 

 been acquired. 



What it is that "directs" a group of animals along a definite "line 

 of specialization" is not known. Darwin's "natural selection" seems 

 to be not fully adequate to account for it, although selection may be an 

 important factor. One theory asserts the action of an internal "drive," 

 of an unknown sort, which holds evolutionary change to a certain 

 direction, as if toward a preordained goal — the theory of ortho- 

 genesis, strongly advocated by the late H. F. Osborn, but not origi- 

 nated by him. 



There is some ground for the idea that specialization may go beyond 

 its optimum point, resulting in a condition of "overspecialization" 

 which handicaps the animal. This is true of increase in size. Up to a 

 certain point, large size combined with corresponding muscular power 



Jmmj 



Fig. 603. The ermine or stoat, Putorius erminea. (Courtesy, Ontario Department 

 of Lands and Forests.) 



