GENERAL GROWTH-GRADIENTS 



105 



towards the front end. In other words, superposed on what- 

 ever growth-mechanisms may be necessary to generate a form 

 similar to that of Diodon, there has arisen a steep and unitary 

 postero-anterior growth-gradient extending throughout the 

 entire body, with high point almost or quite at the extreme 

 hind end. 



The diagram, however, also illustrates the serious limitations 

 of the method. In the first place, 

 two adult forms are contrasted. ,e 



What should rather be contrasted 

 are, phylogenetically, the young 



form of the presumed Diodon-like A \d 



ancestor, and the adult Ortha- 

 goriscus ; or, ontogenetically, a 

 young Diodon-like stage of Ortha- 

 goriscus with the adult condition. 



Fig. 57. — Cartesian transformation of the 



outline of the teleost fish Diodon (left) to 



give the outline of the sun-fish, Ortha- 



goriscus (right). 



But this is not so serious as the following point : that even 

 should you compare the correct two stages, this graphic 

 method, if interpreted in terms of growth, can only give a 

 general and qualitative picture of the mechanism at work, 

 in place of a specific and quantitative one. For if, as D'Arcy 

 Thompson points out, the transformation, so difficult to under- 

 stand at first sight, becomes readily comprehensible on the 

 idea of an orderly change in the distribution of growth-activity 



