CONNECTING AND MISSING LINKS 



aright, for it is the most ancient record of a terrestrial verte- 

 brate known to science, although there is still some differ- 

 ence 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 itet, show conclusively that 

 such was their ancient custom. 



The passage into the next higher group, that of the rep- 

 tiles, 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 tgg, 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 dis- 

 cernible 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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