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In brief, the young chick possesses a series of gill-slits, for these structures are the same in essential plan and relations as the clefts of tadpoles and fishes. Does this mean that even birds have descended from gill-breathing ancestors? Science answers in the affirmative, because evolution gives the only reasonable explanation of such facts as these.
When the latter resumed the gill-breathing habit, this organ lost its original function, and its subsequent history is a curious and significant one. In some modern fishes it has quite disappeared. In others it exists as a minute and useless remnant, no larger than a pea.
For example, frogs and toads develop from tadpoles, which in all essentials are true gill-breathing fish. It is, therefore, obvious that they cannot lay their eggs on dry land, where the tadpoles would be unable to find anything to breathe; so that even the driest and most tree-haunting toads must needs repair to the water once a year to deposit their spawn in its native surroundings.
Other species of the genus became sexually mature for the first time in this condition. This has been regarded as an astounding phenomenon, although every common frog and salamander repeats the metamorphosis in the spring. The whole change from the aquatic and gill-breathing animal to the terrestrial lung-breathing form may be followed step by step in this case.
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