52 • THE RATE OF GROWTH [ch. 



At the same time, we need only consider this part of our 

 subject somewhat briefly. Though it has an essential bearing on 

 the problems of morphology, it is in greater degree involved with 

 physiological problems ; and furthermore, the statistical or 

 numerical aspect of the question is peculiarly adapted for the 

 mathematical 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 characteristics 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 

 characteristic 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 "the form of an 

 organism is determined by its rate of growth in various directions," 

 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 sohd skeletal rod, or when it acts in various 

 other ways which are productive of definite contours, this is a pro- 

 cess of conformation that, both in appearance and reahty, is very 

 difierent 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; and it is in regard to 

 them that we are in a position to study the rate of action in 

 different directions, and to see that it is merely on a difference 

 of velocities that the modification of form essentially depends. 



