60 THE VARIATION OF ANIMALS IN NATURE 



a given area, without any taxonomic specification of the status 

 of the variants it contains. 1 



We are thus presented with very many different kinds of 

 groups, which seem to reflect various modes of divergence in 

 nature and it is desirable to ascertain what is their relationship 

 one with another, and what light they throw on the actual 

 process of divergence itself. 



du Rietz in the paper mentioned above suggests (p. 337) 

 that the most elementary unit of taxonomy is the individual. 

 He points out that the limits of the individual are not always 

 easy to define, but he thinks that the soundest definition 

 involves the recognition of physiological autonomy. We 

 believe, however, that the analysis might be pressed further. 

 To suggest that the character is the most fundamental unit 

 is to open the door to all kinds of complications, chief 

 among which is that the limits of characters are usually 

 very hard to define ; but the suggestion has a particular value 

 from our point of view. Evolution is essentially a matter 

 of character-changes. Individuals are bundles of characters 

 which have each a history of their own, and the divergent 

 groups manifest a progressive accumulation of character- 

 divergences. It is a matter of more than academic or formal 

 interest to keep the individual character before our minds 

 throughout this discussion (cf. lineages, p. 65) and to re- 

 member that the individual maybe resolved into its constituent 

 elements ('structural units' — Swinnerton, 1921, p. 358). 

 The organism has its peculiar autonomy and ' wholeness,' 

 but each of its structural units has an individual history of 

 change which, though related to the needs of the whole or- 

 ganism, can be treated as a separate evolutionary episode. 

 It is also of very great importance to remember the individual 

 character in considering the processes by which we recognise 

 groups of individual organisms such as species, etc. It is 

 not perhaps sufficiently realised how much variation is attain- 

 able, if all the possible characters are taken into account. 

 A. Agassiz (1881, pp. 18-19) pointed out that in the Echinoids 

 the number of variable structural items is at least twenty and 

 that the permutations and combinations of the most restricted 



1 ' Population ' is sometimes used in the sense of ' sample ' in describing local 

 collections made from a larger assemblage. Thus Schmidt (1930, pi. 1) alludes 

 to the population of the Atlantic Cod, though he uses the word ' sample ' in the 

 text. 



