We may summarise, as follows, the main results of the foregoing discussion: (1) Except in certain minute organisms and minute parts of organisms, whose form is due to the direct action of molecular forces, we may look upon the form of the organism as a “function of growth,” or a direct expression of a rate of growth which varies according to its different directions. (2) Rate of growth is subject to definite laws, and the velocities in different directions tend to maintain a _ratio_ which is more or less constant for each specific organism; and to this regularity is due the fact that the form of the organism is in general regular and constant. (3) Nevertheless, the ratio of velocities in different directions is not absolutely constant, but tends to alter or fluctuate in a regular way; and to these progressive changes are due the changes of form which accompany “development,” and the slower changes of form which continue perceptibly in after life. (4) The rate of growth is a function of the age of the organism, it has a maximum somewhat early in life, after which epoch of maximum it slowly declines. (5) The rate of growth is directly affected by temperature, and by other physical conditions. (6) It is markedly affected, in the way of acceleration or retardation, at certain physiological epochs of life, such as birth, puberty, or metamorphosis. (7) Under certain circumstances, growth may be _negative_, the organism growing smaller: and such negative growth is a common accompaniment of metamorphosis, and a frequent accompaniment of old age. (8) The phenomenon of regeneration is associated with a large temporary increase in the rate of growth (or “_acceleration_” of growth) of the injured surface; in other respects, regenerative growth is similar to ordinary growth in all its essential phenomena. ―――――――――― In this discussion of growth, we have left out of account a vast number of processes, or phenomena, by which, in the physiological mechanism of the body, growth is effected and controlled. We have dealt with growth in its relation to magnitude, and to {155} that relativity of magnitudes which constitutes form; and so we have studied it as a phenomenon which stands at the beginning of a morphological, rather than at the end of a physiological enquiry. Under these restrictions, we have treated it as far as possible, or in such fashion as our present knowledge permits, on strictly physical lines.