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al state of great heat and fluidity, perhaps even from a mixture mainly consisting of gases; that such a body as the planet Jupiter represents one of the stages through which it has passed, that such a body as the moon represents a stage toward which it is tending; that it has shrunk as it cooled, and as it shrank has formed the elevations which we call mountains, and the depressions which contain the seas and oceans; that it has been worn by the action of heat from within and water from without, and in consequence of this action presents the appearance when examined below the surface of successive strata or layers; that different kinds of animal and vegetable life have followed one another on the surface, and that some of their remains are found in these strata now; and that all this has taken enormous periods of time. All this is exceedingly probable, because it is the way in which, as Laplace first pointed out, under well-established scientific laws of matter, particularly the law of gravitation and the law of the radiation of heat, a great fluid mass would necessarily change. And the whole solar system may and probably did come into its present condition in this way. It certainly could have been so formed, and there is no reason for supposing that it was formed in any other way. Once more, if we begin, as it were, at the other end, and trace things backwards from the present, instead of forwards from the remote past, it cannot be denied that Darwin's investigations have made it exceedingly probable that the vast variety of plants and animals have sprung from a much smaller number of original forms. In the first place, the unity of plan which can be found pervading any great class of animals or plants seems to point to unity of ancestry. Why, for instance, should the vertebrate animals be formed on a common plan, the parts of the framework being varied from species to species, but the framework as a whole always exhibiting the same fundamental type? If they all descended from a common ancestor, and the variations were introduced in the course of that descent, this remarkable fact is at once accounted for. But, in the second place, observation shows that slight variations ARE perpetually being introduced with every successive generation, and many of these variations are transmitted to the generations that follow. In the course of time, therefore, from any one parent stock would descend a very large variety of kinds.
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