Well, I have just said that in all the gill-breathing Vertebrata, this mechanism of gill-slits and vascular gill-arches in the front part of the intestinal tract is permanent. But in the air-breathing Vertebrata such an arrangement would obviously be of no use. Consequently, the gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here exhibited only as transitory phases of development. But as such they occur in all air-breathing Vertebrata. And, as if to make the homologies as striking as possible, at the time when the gill-slits and the gill-arches are developed in the embryonic young of air-breathing Vertebrata, the heart is constructed upon the fish-like type. That is to say, it is placed far forwards, and, from having been a simple tube as in Worms, is now divided into two chambers, as in Fish. Later on it becomes progressively pushed further back between the developing lungs, while it progressively acquires the three cavities distinctive of Amphibia, and finally the four cavities belonging only to the complete double circulation of Birds and Mammals. Moreover, it has now been satisfactorily shown that the lungs of air-breathing Vertebrata, which are thus destined to supersede the function of gills, are themselves the modified swim-bladder or float, which belongs to Fish. Consequently, all these progressive modifications in the important organs of circulation and respiration in the air-breathing Vertebrata, together make up as complete a history of their aquatic pedigree as it would be possible for the most exacting critic to require. [Illustration: FIG. 54.--Ideal diagram, of primitive gill-or aortic-arches. (After Rathke.) H, outline of heart. The arrows show the course of the blood.] [Illustration: FIG. 55.--The same, modified for a bird. (After Le Conte.) The dark lines show the aortic arches which persist. A, aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids.] [Illustration: FIG. 56.--The same, modified for a mammal. (After Le Conte.)] [Illustration: FIG. 57.--A series of embryos at three comparable and progressive stages of development (marked I, II, III), representing each of the classes of vertebrated animals below the Mammalia (After Häckel.)] [Illustration: FIG. 58.--Another series of embryos, also at three comparable and progressive stages of development (marked I, II, III), representing four different divisions of the class Mammalia. (After Häckel.)]