constitutes Paleontology. This science has shown that forms so different as the Horse and Rhinoceros are linked together by the fossil forms of Coryphodon, Paleotherium, etc.; that the Pig and Hippopotamus represent another group, connected by Anoplotherium and Dichotrum, etc.; that the Fishes and Batrachia form one great division, the Reptiles and Birds another,—forms linking indissolubly together these divisions of the Vertebrates having been discovered in different parts of the world. The structure, the development, and fossil remains all harmonize in proving without a doubt that they are only the modified posterity of a class now extinct, which the Amphioxus nearly represents. That the Amphioxus came from Worms in their structure allied to Ascidians, is highly probable. But, given the Amphioxus, that the genealogy of the Vertebrata is represented by a tree, like that provisionally offered in Tree V., seems to us to follow, without doubt, from the facts of Anatomy, Geology, and Embryology.
FISHES.
The Amphioxus, or Lancelet, is a little animal about two inches long, found generally buried in the sand on the coasts of different seas. It does not possess head, brain, eyes, or limbs, and yet there exists a backbone in a rudimentary condition (notochord), and marrow. Its gills are not like those of fishes, but its branchial apparatus is that of an Ascidian (Fig. 40), confirming the view of the origin of the Amphioxus from the Ascidian Worms, suggested by their identical development. What is the Amphioxus? It seems to be an intermediate animal, a link connecting the Ascidian with the Fishes. The part of the body containing the mouth is usually regarded as the head, and is membranous in structure, which condition is found in fishes at certain periods of their existence. We may designate the
