a phenomenon to be referred in part to the direct action of molecular forces, in part to a more complex and slower process, indirectly resulting from chemical, osmotic and other forces, by which material is introduced into the organism and transferred from one part of it to another. It is this latter complex phenomenon which we usually speak of as ãgrowth.ã {54}
Every growing organism, and every part of such a growing organism, has its own specific rate of growth, referred to a particular direction. It is the ratio between the rates of growth in various directions by which we must account for the external forms of all, save certain very minute, organisms. This ratio between rates of growth in various directions may sometimes be of a simple kind, as when it results in the mathôÙeôÙmatôÙiôÙcally definable outline of a shell, or in the smooth curve of the margin of a leaf. It may sometimes be a very constant one, in which case the organism, while growing in bulk, suffers little or no perceptible change in form; but such equiôÙlibôÙrium seldom endures for more than a season, and when the ratio tends to alter, then we have the phenomenon of morphological ãdevelopment,ã or steady and persistent change of form.
This elementary concept of Form, as determined by varying rates of Growth, was clearly apprehended by the mathôÙeôÙmatôÙiôÙcal mind of Haller,ãwho had learned his mathematics of the great John Bernoulli, as the latter in turn had learned his physiology from the writings of Borelli. Indeed it was this very point, the apparently unlimited extent to which, in the development of the chick, inequalities of growth could and did produce changes of form and changes of anatomical ãstructure,ã that led Haller to surmise that the process was actually without limits, and that all development was but an unfolding, or ãevolutio,ã in which no part came into being which had not essentially existed before4. In short the celebrated doctrine of ãpreformationã implied on the one hand a clear recognition of what, throughout the