Page:Popular Science Monthly Volume 77.djvu/157

Rh and character in the ancestral type at the same stage of its development, it represents an actual repetition of past history and is therefore palingenetic. Sometimes it is not quite clear, however, under which caption the embryonic structure comes, and its interpretation must be attempted with caution.

Osborn in his lectures to his students speaks of the three-fold evidence for evolution which stands firmly like a tripod, the legs of which are comparative anatomy, embryology and paleontology; and the evidence of each should correspond, provided the interpretation be correct. Of these, however, embryology is manifestly the weakest member, while paleontology is a tower of strength!

The reptiles are so rare as embryos and withal so ancient a group that their ontogeny throws but little light upon paleontology. Among the fossil forms a number of specimens of Ichthyosaurus have been found with young contained within the body of the adult. Many of these are in the normal position for fœti-in-utero; others are displaced, with the head directed forward. These latter Branca thinks may be young that have been eaten. There is also, at times, a very great difference in the size of the contained young. Aside from a slight difference in proportions, especially that of head to trunk, and a less degree of hardness of the embryonic bones, as indicated by their being crushed over the parent's ribs, the young teach us nothing as to ancestral structure as they are in every way perfect ichthyosaurs. They do prove, however, when the evidence of viviparity which they offer is taken in connection with the supreme degree of aquatic adaptation indicated, that the ichthyosaurs were high sea-forms, never coming ashore even for egg-laying.

That certain of the dinosaurs were also viviparous may be proved by an embryo contained in the unique specimen of Compsognathus longipes from the Jurassic of Bavaria. So far as I am aware this embryo gives no other evidence of ontogenetic value.

The turtles have been made the subject of exhaustive study by Hay and from the embryological point of view by Clark under L. Agassiz. Anatomically they are the most remarkable of reptiles, having undergone during their career an extreme modification in many directions while retaining a number of very primitive characters. The most remarkable feature is the development of the shield or carapace, which contains what are generally considered as the homologues of the ribs of other vertebrates, but, strangely enough, here lying outside of the shoulder girdle, a feature wherein the turtles are utterly unique. The embryology, which is well known, ought to throw some light upon the origin of this important feature. In their earlier stages of development the Chelonia resemble the Lacertilia, the chief pecularity being caused by the development of this carapace which appears in the form