for the mathôÙeôÙmatôÙiôÙcal study of variation and correlation. On these important subjects we shall scarcely touch; for our main purpose will be sufficiently served if we consider the charôÙacôÙterisôÙtics of a rate of growth in a few illustrative cases, and recognise that this rate of growth is a very important specific property, with its own charôÙacôÙterôÙisôÙtic value in this organism or that, in this or that part of each organism, and in this or that phase of its existence.
The statement which we have just made that ãthe form of an organism is determined by its rate of growth in various directions,ã is one which calls (as we have partly seen in the foregoing chapter) for further explanation and for some measure of qualification. Among organic forms we shall have frequent occasion to see that form is in many cases due to the immediate or direct action of certain molecular forces, of which surface-tension is that which plays the greatest part. Now when surface-tension (for instance) causes a minute semi-fluid organism to assume a spherical form, or gives the form of a catenary or an elastic curve to a film of protoplasm in contact with some solid skeletal rod, or when it acts in various other ways which are productive of definite contours, this is a process of conformation that, both in appearance and reality, is very different from the process by which an ordinary plant or animal grows into its specific form. In both cases, change of form is brought about by the movement of portions of matter, and in both cases it is ultimately due to the action of molecular forces; but in the one case the movements of the particles of matter lie for the most part within molecular range, while in the other we have to deal chiefly with the transference of portions of matter into the system from without, and from one widely distant part of the organism to another. It is to this latter class of phenomena that we usually restrict the term growth;