. The chordates. Chordata. 790 Comparative Morphology i' Chordates ; 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 "; Snak
. The chordates. Chordata. 790 Comparative Morphology i' Chordates ; 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 "; 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
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