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matter of observable fact that all plants and animals are perpetually engaged in what Mr. Darwin calls a "struggle for existence." That is to say, in every generation of every species a great many more individuals are born than can possibly survive; so that there is in consequence a perpetual battle for life going on among all the constituent individuals of any given generation. Now, in this struggle for existence, which individuals will be victorious and live? Assuredly those which are best fitted to live the weakest and the least fitted to live will succumb and die, while the strongest and the best fitted to live will be triumphant and survive. Now it is this "survival of the fittest" that Mr. Darwin calls "natural selection." Nature, so to speak, selects the best individuals out of each generation to live. And not only so, but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed
laws of heredity, it follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers. And this follows, not merely because in every generation it is only the flower of the race that is allowed to breed, but also because if in any generation some new and beneficial qualities happen to appear as slight variations from the ancestral type, these will be seized upon by natural selection and added, by transmission in subsequent generations, to the previously existing type. Thus the best idea of the whole process will be gained by comparing it with the closely analogous process whereby gardeners and cattlebreeders create their wonderful productions; for just as these men, by always selecting their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similar process of selection, slowly but
continuously makes the various species of plants and animals better and better suited to the external conditions of their life.
Now, if this process of continuously adapting organisms to their environment takes place in nature at all, there is no reason why we should set any limits on the extent to which it is able to go up to the point at which a complete and perfect adaptation is achieved. Therefore we might suppose that all species would attain to this condition of perfect adjustment to their environment, and there remain fixed. And so undoubtedly they would, if the environment were itself unchanging. But forasmuch as the environment-or the sum total of the external conditions of life of almost every organic type alters more or less from century to century (whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations of other species living on contiguous
geographical areas), it follows that the process of natural selection need never reach a terminal phase. And forasmuch as natural selection may thus continue, ad infinitum, slowly to alter a specific type in adaptation to a gradually changing environment, if in any case the alteration thus effected is sufficient in amount to lead naturalists to denote the specific type by some different name, it follows that natural selection has transmuted one specific type into another. And so the process is supposed to go on over all the countless species of plants and animals simultaneously-the world of organic types being thus regarded as in a state of perpetual, though gradual, flux.
Such, then, is the theory of natural selection, or survival of the fittest; and the first thing we have to notice with regard to it is, that it offers to our acceptance a scientific explanation of the numberless cases of apparent design which we
everywhere meet with in organic nature. For all such cases of apparent design consist only in the adaptation which is shown by organisms to their environment, and it is obvious that the facts are covered by the theory of natural selection no less completely than they are covered by the theory of intelligent design. Perhaps it may be answered, "The fact that these innumerable cases of adaptation may be accounted for by natural selection is no proof that they are not really due to intelligent design." And, in truth, this is an objection which is often urged by minds -even highly cultured minds-which have not been accustomed to scientific modes of thought. I have heard an eminent professor tell his class that the many instances of adaptation which Mr. Darwin discovered and described as occurring in orchids, seemed to him to tell more in favour of contrivance than in favour of natural causes; and another eminent professor once