Let it be supposed, then, that there was a period in the history of this earth when the whole human race, however it originated, was confined to an island, thousands of miles from any other land. This race of men adapted to a life in one medium, the air, may be supposed to have so far advanced in the ruder arts of hunting and fishing, and in the higher art of tillage, as to be able for many generations to support life by what the sea and the land would put within their reach, and by the product which their rude agriculture could extract from the soil, or which the soil would spontaneously yield. But as the centuries flow on, the population begins to press upon the resources of the territory, and the struggle for life becomes very great. At length a point is reached where the supply of food from the land becomes inadequate to sustain the population, and what can be made up from the sea will not supply the deficiency. The population will then slowly decrease, but, while this decrease goes on, there comes in a disturbing cause which will prevent any adjustment of the supply of food to the diminished number of the consumers. The sea begins by almost imperceptible but steadily progressing encroachments to diminish the area of dry land; a change of climate reduces the number of other animals available for human food, and reduces the productive capacity of the earth. Then ensues that struggle for existence which is supposed to entail changes of medium of life, and to induce transformations of structure. The conditions of existence have become wholly changed. The wretched descendants of a once comparatively thriving race are dwelling on a territory which has become a marsh. They have no means of migrating to another territory; they can only migrate to another medium. They begin by feeding exclusively on what the water will afford. They pass their lives in the pursuit of a prey which lives only in the water, and in this change of life they acquire or develop organs adapted to the new condition, organs which, in such miserable reproduction of their own species as can go on, they transmit to their offspring. Modifications upon modifications accumulate in this way through untold periods of time, until at last a new aquatic or a new amphibious creature is formed, and the difference between that creature and his remote ancestral human stock is as great as that between man and the seal, or between man and any fish that swims. Still, there will be peculiarities of structure retained, which might lead any inhabitant of another world, alighting on this globe and undertaking to trace the origin of this new creature, to the supposition that he was akin to a race of men whose fossil remains he might find buried in some stratum beneath the marsh which was the last habitat of this unfortunate race, when it had all the characteristics of its original type.