CONNECTING AND MISSING LINKS
aright, for it is the most ancient record of a terrestrial vertebrate known to science, although there is still some difference of opinion as to its correct interpretation. But it is baffling in its obscurity, as it tells little of the structure of the impressed foot and nothing at all of any other part of the owner’s frame, except, again, by inference. This footprint (Thinopus) is a most important link in a weak part of our chain; but other links, which thus far are known but vaguely or not at all, are necessary for the chain’s integrity.
The old-time emergents—those that first left the water—were lung-breathers, of course, but they still went back to the water to bring forth their young. Living amphibians, such as frogs and salamanders, yet do this each recurring season, and gill-arches preserved in certain fossils, along with shore-adapted hands and feet, show conclusively that such was their ancient custom.
The passage into the next higher group, that of the reptiles, implies the loss of gill-breathing in the young and the consequent laying of eggs ashore. This passage has given rise to a large, complex egg, which can both nourish the developing embryo and allow it to breathe as well, until at the time of hatching it emerges as a miniature snake or turtle or crocodile, according to its kind, but never as a form reminiscent of the ancestral fish in shape or habit. Here the transitional types are surely known; the uninitiated can not tell whether they are amphibian or reptile, the difference lying in certain technical details of structure that are discernible only to the expert; and here, therefore, our chain has all the requisite strength of continuity.
Links of great interest are those that connect reptile and bird on the one hand and reptile and mammal on the other. Huxley, years ago, spoke of the birds as “glorified reptiles,” and his descriptive term is still very apt, for the birds are
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