=2. Aortic Arches.=--But some will, perhaps, say that these stages in the ontogeny are only examples of adaptive modifications--like modifications for like conditions of life--and had better be accounted for in this way, without reference to family history. We will, therefore, take another example, which can not be thus accounted for--an example in which there is no possible use _now_ for the peculiar form or structure which we find. For this purpose we take the case of the _course of circulation in vertebrates_. [Illustration: FIG. 36.--Showing heart and outgoing blood-vessels of a lizard (after Owen). The arrows show the course of the blood.] [Illustration: FIG. 37.--A, heart and gill-arches of a fish; B, one arch, with fringe (after Owen); H, the heart.] If one examines the large vessels _going out_ from the heart of a lizard, he will find _six aortic arches_--i. e., three on each side. These all unite below to form the one descending abdominal aorta. This is shown in the accompanying figure (Fig. 36), in which _a a′ a″_ and _b b′ b″_ are the six arches. Now, there is no conceivable use in having so many aortic arches. We know this, because there is but one in birds and mammals, and the circulation is as effective, nay, much more effective in these than in reptiles. The explanation of this anomaly is revealed at once as soon as we examine the circulation of a _fish_, which is shown in the accompanying figure (Fig. 37). The multiplication of the aortic arches is here, of course, necessary, for they are the _gill-arches_. The whole of the blood passes through these arches, to be aërated in the gill-fringes. The use of this peculiar structure is here obvious enough. If a lizard were ever a fish, and afterward turned into a lizard, changing its gill-respiration for lung-respiration, then, of course, the useless gill-arches would remain to tell the story. Now, although a lizard never was a fish, in its _individual_ history or ontogeny, it was a fish in its family history or phylogeny, and therefore it yet retains, by heredity, this curious and _useless_ structure as evidence of its ancestry. [Illustration: FIG. 38.] [Illustration: FIG. 39. FIGS. 38, 39.--Diagrams showing the change of the course of blood in the development of a frog. 38. The tadpole stage. 39. The mature condition. H, heart; G G′ G″, external gills; _g g′ g″_, internal gills; _c c_, connecting branches in the tadpole; _p p_, pulmonary branches. ]