There are several species of tree-frogs, and toads, and closely related amphibia like the salamanders, that do not lay their eggs in the water. Some of them bury them in folds of their own external skin, others (such as the Alpine salamander) retain them within the mother’s body, as the mammals do. The young animals develop there from the eggs. Even there, however, where there is no question of aquatic life, the young frogs, toads, and salamanders first assume the fish-form. The young frogs and toads have fin-like tails, and all of them have gills. There seems to be some _internal_ law of development that forces the frog and its relatives to pass through the fish-stage in their individual evolution even when there is no trace whatever of any external utility. Now let us examine the matter as Darwinians and believers in evolution. There are reasons on every hand for believing that the frogs and salamanders, which now stand higher in classification than the fishes, were developed from the fishes in earlier ages in the course of progressive evolution. Once upon a time they were fishes. If that is so, the curious phenomenon we have been considering really means that each young frog resembles its _fish-ancestors_. In each case to-day the frog’s egg first produces the earlier or ancestral stage, the fish. It then develops rapidly into a frog. In other words, the individual development recapitulates an important chapter of the earlier history of the whole race of frogs. Putting this in the form of a law, it runs: each new individual must, in its development, pass rapidly through the form of its parents’ ancestors before it assumes the parent form itself. If a new individual frog is to be developed, and if the ancestors of the whole frog-stem were fishes, the first thing to develop from the frog’s egg will be a fish, and it will only later assume the form of a frog. That is a simple and pictorial outline of what we mean when we speak of “the biogenetic law.” We need, of course, much more than the one frog-fish fact before we can erect it into a law. But we have only to look round us, and we find similar phenomena as common as pebbles. Let us bear in mind that evolution proceeded from certain amphibia to the lizards, and from these to the birds and mammals. That is a long journey, but we have no alternative. If the amphibia (such as the frog and the salamander) descend from the fishes, all the higher classes up to man himself must also have done so. Hence the law must have transmitted even to ourselves this ancestral form of the gill-breathing fish.