I am disposed to think that the prominence of this conception in Aristotle’s theory of things arose from the circumstance that he was to begin with and throughout his life, devoted to biological studies.  In fact it is a notion which must force itself upon the mind of any one who studies biological phenomena, without reference to general physics, as they now stand.  Everybody who observes the obvious phenomena of the development of a seed into a tree, or of an egg into an animal, will note that a relatively formless mass of matter gradually grows, takes a definite shape and structure, and, finally, begins to perform actions which contribute towards a certain end, namely, the maintenance of the individual in the first place, and of the species in the second.  Starting from the axiom that every event has a cause, we have here the causa finalis manifested in the last set of phenomena, the causa materialis and formalis in the first, while the existence of a causa efficiens within the seed or egg and its product, is a corollary from the phenomena of growth and metamorphosis, which proceed in unbroken succession and make up the life of the animal or plant.

Thus, at starting, the egg or seed is matter having a “form” like all other material bodies.  But this form has the peculiarity, in contradistinction to lower substantial “forms,” that it is a power which constantly works towards an end by means of living organisation.

So far as I know, Leibnitz is the only philosopher (at the same time a man of science, in the modern sense, of the first rank) who has noted that the modern conception of Force, as a sort of atmosphere enveloping the particles of bodies, and having potential or actual activity, is simply a new name for the Aristotelian Form.[19] In modern biology, up till within quite recent times, the Aristotelian conception held undisputed sway; living matter was endowed with “vital force,” and that accounted for everything.  Whosoever was not satisfied with that explanation was treated to that very “plain argument”—­“confound you eternally”—­wherewith Lord Peter overcomes the doubts of his brothers in the “Tale of a Tub.”  “Materialist” was the mildest term applied to him—­fortunate if he escaped pelting with “infidel” and “atheist.”  There may be scientific Rip Van Winkles about, who still hold by vital force; but among those biologists who have not been asleep for the last quarter of a century “vital force” no longer figures in the vocabulary of science.  It is a patent survival of realism; the generalisation from experience that all living bodies exhibit certain activities of a definite character is made the basis of the notion that every living body contains an entity, “vital force,” which is assumed to be the cause of those activities.