GENERAL GROWTH-GRADIENTS 



107 



mechanism by which growth-changes must operate, then you 

 can deduce the precise course of evolution within comparatively 

 narrow limits (see Fig. 59). 



As showing the validity of the method, D'Arcy Thompson 

 has applied it to the evolution of the skeleton of the horse, 



Fig. 58. — Carapaces of various crabs to show how they may be readily derived 

 from one form by simple Cartesian transformation. 



and then compared his deduced results with the known fossil 

 forms (Fig. 60). 



The agreement of actual with deduced skull-form is very 

 close for Protohippus, Miohippus and Mesohippus. But the 

 species of Parahippus figured by Thompson, as he points out, 



