EVOLUTION OF THE VERTEBRATES 149 



accomplish the shift of habitat, these ancestral forms must have 

 been able to breathe air, and because of the extreme changes in 

 locomotion, they must have had structures adequate to move 

 their bodies over a solid substratum. The other conditions of 

 change need not have been mot at once. If the primitive terres- 

 trial vertebrates remained in a moist environment, as would be 

 expected, protection against dcssication would be unnecessary. 

 Changes in the special senses and in the skeleton would be valu- 

 able, but in the absence of competition with other terrestrial 

 species, would not necessarily be of vital importance. This transi- 

 tional state is nicely illustrated by the existing Amphibia, par- 

 ticularly the tailed species, the newts and salamanders. 



When we seek fishes with some capacity for terrestrial life, we 

 find that two groups are capable of breathing air by a diverticulum 

 of the alimentary tract fundamentally similar to a primitive verte- 

 brate lung. While other fishes are able to exist out of water, 

 the blennies even to the extent of leaving it to escape their enemies, 

 they are adapted in ways different from the usual terrestrial 

 forms. One of the two significant groups is the subclass Dipnoi, 

 the lung-fishes. The other is the order Crossopterygii of the sub- 

 class Teleostomi. The Dipnoi are an interesting group but for 

 the purposes of this study they may be set aside. Their paired fins, 

 which alone are in a position to aid in locomotion on a solid sub- 

 stratum, are very different in structure from the pentadactyl 

 appendage and cannot be looked upon as precursors of such ap- 

 pendages. 



The Crossopterygii. In the order Crossopterygii, on the other 

 hand, the structure of the pectoral fins shows a surprising degree 

 of resemblance to the terrestrial liml). The fishes of this order use 

 the pectoral fins not merely as l^alancing organs, but also as paddles 

 in swimming, and when resting on the bottom as support for the 

 anterior part of the body. Like the pentadactyl appendage they 

 have a single proximal bone, two bones distal to it, and several 

 series of smaller bones distal to the two. To the small bones is 

 attached the fringe of dermal rays which support the aquatic part 

 of the appendage. However the fins are not the only point of 

 resemblance. The fishes breathe air at the surface of the water 

 by means of a double air sac connected by a single tube to the 

 ventral side of the pharynx, a condition resembling the lungs of 

 Amphibia and the early embryonic lungs of other classes. The 



