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 eventually reach this condition of perfect harmony with their environment, and then remain fixed. And so, according to the theory, they would, if the environment were itself unchanging. But forasmuch as the environment (i. e. 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 areas, and so on--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 name the result as a distinct species, it follows that natural selection has transmuted one specific type into another. Similarly, by a continuation of the process, specific types would become transmuted into generic, generic into family types, and so on. Thus the process is supposed to go on throughout all the countless forms of life continuously and simultaneously--the world of organic types being thus regarded as in a state of perpetual, though gradual, flux. * * * * *