ORGANIC SYMMETRY n 



Phylogeny and heredity illustrate different 

 phases of the theory of descent with modifica- 

 tion, and have to be approached by appropriate 

 methods. The test of breeding, which is crucial 

 in heredity, is obviously inapplicable in a phylo- 

 genetic investigation where the last positive links 

 in the chain of evidence can, as a rule, only be 

 afforded, if at all, by palaeontology. 



Morphology, in its aesthetic aspect, is the 

 perception of symmetry and organic beauty. Its 

 interest and importance lie less in its ultimate 

 truth than in its relative completeness, by which 

 I mean its capacity for embracing the widest 

 possible range of facts without ignoring those 

 which may not fall into harmony with a parti- 

 cular theory. 



The distinction between convergent and normal 

 morphogeny has long pervaded biological litera- 

 ture ; but I believe it to be a fact that the 

 equality of interest which attaches to the two 

 branches of the subject has not been recognised 

 in an adequate manner hitherto. It will appear 

 in the course of this essay that convergence is 

 neither identical with homoplasy l nor with ceno- 

 genesis, 2 but that it includes these and some- 



1 Homoplasy signifies similarity of form unaccompanied by 

 community of pedigree. As explained below, we owe the term 

 to Sir Ray Lankester. 



2 Cenogenesis implies the origin of structural features by 

 relatively recent adaptation, in contrast with palingenesis or 

 primordial adaptation. The terms are due to Professor Haeckel. 



