There are particular habitats in which animals are subject to changes of media. In such habitats exist animals having, in various degrees, the power to live in both media, consequent on various phases of transitional organization. Near akin to these animals, there are some that, after passing their early lives in the water, acquire more completely the structures fitting them to live on land, to which they then migrate. Lastly, we have closely-allied creatures like the Surinam toad and the terrestrial salamander, which, though they belong by their structures to the class Amphibia, are not amphibious in their habits--creatures the larvæ of which do not pass their early lives in the water, and yet go through these same metamorphoses! Must we, then, think that the distribution of kindred organisms through different media presents an insurmountable difficulty? On the contrary, with facts like these before us, the evolution-hypothesis supplies possible interpretations of many phenomena that are else unaccountable. Realizing the way in which such changes of media are in some cases gradually imposed by physical conditions, and in other cases voluntarily commenced and slowly increased in the search after food, we shall begin to understand how, in the course of evolution, there have arisen those strange obscurations of one type by the externals of another type. When we see land-birds occasionally feeding by the water-side, and then learn that one of them, the water-ouzel, an "anomalous member of the strictly terrestrial thrush family, wholly subsists by diving--grasping the stones with its feet and using its wings under water"--we are enabled to comprehend how, under pressure of population, aquatic habits may be acquired by creatures organized for aërial life; and how there may eventually arise an ornithic type, in which the traits of the bird are very much disguised. Finding among mammals some that, in search of prey or shelter, have taken to the water in various degrees, we shall cease to be perplexed on discovering the mammalian structure hidden under a fish-like form, as it is in the Cetacea. Grant that there has even been going on that redistribution of organisms which we see still resulting from their intrusions on one another's areas, media, and modes of life, and we have an explanation of those multitudinous cases in which homologies of structure are complicated with analogies. And while it accounts for the occurrence, in one medium of organic types fundamentally organized for another medium, the doctrine of evolution accounts also for the accompanying unfitness. Either the seal has descended from some mammal which, little by little, became aquatic in its habits, in which case the structure of its hind-limbs has a meaning; or else it was specially framed for its present habitat, in which case the structure of its hind-limbs is incomprehensible.[90]