Naturally enough, it is from the biological sciences, in which the study of organic _growth_ plays so prominent a part, that the impulse to conceive of physical change as development originally comes. As long ago as the fourth century B.C., Aristotle had taken the concept of growth or development as the foundation of the most influential scheme of metaphysical construction yet produced in the whole history of speculation. In Aristotle’s view, however, the process of development was regarded as strictly confined within the limits of the individual life. The individual organism, beginning its existence as an undeveloped germ or potentiality, gradually unfolds itself in a series of successive stages of growth, which culminates at the period of complete maturity. But the individual germ itself is a product or secretion derived from a pre-existing mature individual of the same type as that into which this germ will ultimately grow. The number of distinct typical processes of growth is thus strictly determined, and each such process implies the previous existence of its completed result. In other words, the boundaries between _species_ are fixed and ultimate; there can be no beginning in time of the existence of a new species, and therefore no origination of new species by development from other types. As Aristotle epigrammatically puts it, “It takes a man to beget a man.” A further point of weakness in the Aristotelian theory is the absence of any definite account of the _machinery_ by which the process of growth is effected. We learn, indeed, that the latent capacity of the organic germ to develop according to a certain specific type is stimulated into activity by influences contained in the environment, but the precise nature of this process of stimulation was necessarily left in obscurity, in consequence of the imperfect knowledge possessed by Aristotle of the minute character of natural processes in general.