80 THE RATE OF GROWTH [ch. 



At the same time, we need only consider this large 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; also, the statistical or numerical aspect of 

 the question is peculiarly adapted to the mathematical study of 

 variation and correlation. These important subjects we must not 

 neglect; but our main purpose will be 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 for further explanation and for some measure of 

 quahfication. 



Among organic forms we shall have many an occasion to see that 

 form may be due in simple cases to the direct action of certain 

 molecular forces, among which surface-tension plays a leading part. 

 Now when surface-tension causes (for instance) a minute semifluid 

 organism to assume a spherical form, or gives to a film of protoplasm 

 the form of a catenary or of an elastic curve, or when it acts in 

 various other ways productive of definite contours — just as it does 

 in the making of a drop, a splash or a jet — this is a process of con- 

 formation very diiferent from that 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 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; it is in regard to them that we are in a 

 position to study the rate of action in different directions and at 

 different times, and to realise that it is on such differences of rate 

 that form and its modifications essentially and ultimately depend. 



