CHAPTER IV 



GROWTH-GRADIENTS AND THE GENERAL 

 DISTRIBUTION OF GROWTH-POTENTIAL 

 IN THE ANIMAL BODY 



§ i. General Growth-gradients : D'Arcy Thompson's 



Graphic Method 



THE next step is to inquire whether the growth- 

 gradient mechanism may not underlie the general 

 growth of the body as well as the growth of specialized 

 appendages or regions. In other words, we want to see whether 

 the sharply-marked growth-gradients of a chela or an abdomen 

 are not merely special cases of a more general but still orderly 

 mechanism underlying the distribution of what we may, for 

 want of a better term, call growth-potential, throughout the 

 whole organism. 



A strong indication that this is so was afforded by D'Arcy 

 Thompson (1. c, chap. 17) through his ingenious application of 

 the principle of Cartesian co-ordinates to the problem of 

 animal form. Let us take the most spectacular and at the 

 same time one of the simplest of his instances. The strange 

 fantastic sun-fish, Orthagoriscus, is a close relative of such 

 types as Diodon ; but its adaptation to an almost planktonic 

 life at the surface of the sea has led to a change of form so 

 radical that it is at first sight difficult to see how it could have 

 been brought about within any short space of evolutionary 

 time. D'Arcy Thompson, however, pointed out that if you 

 inscribed the outline of a Diodon in a framework of rectangular 

 co-ordinates, and then distorted this in a certain perfectly 

 regular way, you would obtain a very close approximation of 

 the outline of an Orthagoriscus (Fig. 57). From the figure 

 it will be immediately obvious that the essence of the trans- 

 formation, considered biologically and not merely as an 

 exercise in higher geometry, must have been the origin of a 

 very active growth-centre in the whole of the hind-region of 

 the body, whence the intensity of growth diminished regularly 



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