60 THE RATE OF GROWTH [ch. 



homogeneous to exert at every point an approximately uniform 

 resistance. Under these conditions then, the rate of growth is 

 uniform in all directions, and does not affect the form of the 

 organism. 



But in a larger or a more complex organism the study of growth, 

 and of the rate of growth, presents us with a variety of problems, 

 and the whole phenomenon becomes a factor of great morphological 

 importance. • We no longer find that it tends to be miiform in 

 all directions, nor have we any right to expect that it should. 

 The resistances which it meets with will no longer be uniform. 

 In one direction but not in others it will be opposed by the 

 important resistance of gravity; and within the growing system 

 itself all manner of structural differences will come into play, 

 setting up unequal resistances to growth by the varying rigidity 

 or viscosity of the material substance in one direction or another. 

 At the same time, the actual sources of growth, the chemical and 

 osmotic forces which lead to the intussusception of new matter, 

 are not uniformly distributed ; one tissue or one organ may well 

 manifest a tendency to increase while another does not; a series 

 of bones, their intervening cartilages, and their surrounding 

 muscles, may all be capable of very different rates of increment. 

 The differences of form which are the resultants of these differences 

 in rate of growth are especially manifested during that, part of 

 life when growth itself is rapid: when the organism, as we say, 

 is undergoing itfe develojoment . When growth in general has 

 become slow, the relative differences in rate between different 

 parts of the organism may still exist, and may be made manifest 

 by careful observation, but in many, or perhaps in most cases, the 

 resultant change of form does not strike the eye. Great as are 

 the differences between the rates of growth in different parts of 

 an organism, the marvel is that the ratios between them are so 

 nicely balanced as they actually are, and so capable, accordingly, 

 of keeping for long periods of time the form of the growing organism 

 all but unchanged. There is the nicest possible balance of forces 

 and resistances in every part of the complex body; and when 

 this normal equilibrium is disturbed, then we get abnormal 

 growth, in the shape of tumours, exostoses, and malformations 

 of every kind. 



