6. It is only on the supposition that the life of the earth has evolved step by step with the evolution of the land masses, and that the forms of life from which existing forms were evolved were dispersed over the earth at a time when physiographic conditions were very different from what they are now, that it is possible to account for the peculiar manner in which animals are distributed over the earth. The cassowary is a flightless bird of the ostrich order inhabiting Australia and the islands to the north of it. This bird is found nowhere else in the world, and each area has its own particular species. The same things are also true of the kangaroo. It is found over a similar region, with a different species occupying each land mass. Now, on the hypothesis of special creation there is no thinkable reason why these animals should be divided, as they are, into distinct species, and restricted to this particular region. But on the hypothesis of evolution it is perfectly plain. All of these regions at one time were united with one another, and were subsequently submerged in part, forming islands. Each group of animals, being isolated from every other group and subjected to somewhat different conditions, developed a style of departure from the original type of structure different from that of every other group in response to the peculiar conditions operating upon it. This has led, in the course of centuries of selection, to the formation of distinct species such as exist to-day.