324 UNITY AND DIVERSITY IN BIOCHEMISTRY 



the first half of the 19th century, this idea appeared in numerous guises 

 depending on the personal preferences of its protagonists. For the most 

 theoretically inclined, like Goethe or Oken "these ideal patterns which the 

 creative principle set before itself were, so to say, Platonic ideas in the mind 

 of the creative spirit" (Sir Charles Sherrington. Goethe on Nature and on 

 Science. Cambridge, 1949, p. 24). The more objective, as Julian Huxley 

 said of Thomas Huxley, "simply assumed that structural homology (or 

 common archetypal plan) was the right key to unlock classificatory secrets" 

 (Julian Huxley. Evolution. The modern synthesis, London, 1942, p. 391). We 

 might as well say that during the first half of the 19th century, the idea 

 of a plan was purely descriptive and had no explanatory basis, and that 

 Schiller was right when he repHed to the exposition of Goethe : "Das ist 

 keine Erfahrung; das ist eine Idee." 



The publication of the great work by Charles Darwin changed all this. 

 Since the advent and generalization of the Evolution Theory, the basic 

 criterion for natural classification, which up till then was the degree of simi- 

 larity, has become the degree of phylogenetic kinship. The aim of present 

 day taxonomy is to build up a classification consisting of classes based upon 

 phylogenetic relations. As Dobhzansky has pointed out, this new point of view 

 has not led to any fundamental modification of classification. Here is an 

 important point, the consideration of which leads to an increased confidence 

 in the system established by taxonomists, which we shall use as our guide. 



When discussing the significance of various characteristics of plants useful 

 in their taxonomy at the supraspecific level, Erdtmann (1952) wrote: "Much 

 attention is paid by taxonomists to minor characteristics such as the form, 

 structure and arrangement of epidermal cells because they are essentially 

 indifferent to external factors and consequently conservative. Such proper- 

 ties are therefore handed down from generation to generation and from 

 species without suffering much change. Comparatively recent specializa- 

 tions possess little taxonomic interest." 



From the biochemical aspect, as with other aspects, if it is true that the 

 characteristics of the species and its sub-divisions are naturally inserted into 

 the physiology of the organism and its relation with its ecological niche, it 

 is to be expected that the biochemical traits which are transmitted from 

 organism to organism and bear witness to a common line of ancestors will 

 not be those of an essential physiological or ecological character. The 

 utility of supraspecific categories is above all to enable us to express our 

 views on the probable nature of phylogeny. As Caiman (1949) says, "the 

 characters most important in taxonomy are those which maintain them- 

 selves unchanged through the greatest range of variation." 



The higher categories will thus indicate those limits within which 

 characters common to a more or less broad range of species maintain them- 

 selves in spite of great species, physiological and ecological differences. The 



