586 THE POPULAR SCIENCE MONTHLY 



the other evidence to be presented) that we may regard the latter system 

 in the same light. 



Another peculiarity of organic classification, which, as shown by 

 Spencer, is important because of its indication of evolution, is the var- 

 iable degree of differentiation between corresponding groups and sub- 

 groups. 



. . . The successively subordinate classes, orders, genera and species, into 

 which zoologists and botanists segregate animals and plants have not, in reality, 

 those definite values conventionally given to them. There are well-marked 

 species, and species so imperfectly defined that certain systematists regard them 

 as varieties. Between genera, strong contrasts exist in many cases; and in other 

 cases, contrasts so much less decided as to leave it doubtful whether they con- 

 stitute generic distinctions. So, too, it is with orders and classes; in some of 

 which there have been introduced intermediate sub-divisions having no equiva- 

 lents in others. Even of the sub-kingdoms the same truth holds. The contrast 

 between the Molluscoida and the Mollusca is far less than between the Mollusca 

 and the Annulosa, and there are naturalists who think that the vertebrata are 

 so much more widely separated from the other subkingdoms, than these are from 

 one another, that the Vertebrata should have a classificatory value equal to that 

 of all the other subkingdoms taken together. 6 



Although at first thought this peculiarity may not seem to be of 

 much importance, yet Spencer showed, by comparison with the case of 

 languages, in which exactly analogous characteristics are observable, 

 and in which evolution is known to have taken place, that it is an 

 additional indication of evolution. 7 



If, then, the classification of organisms results in several orders of assem- 

 blages, such that assemblages of the same order are but indefinitely equivalent; 

 and if, where evolution is known to have taken place, there have arisen assem- 

 blages between which the equivalence is similarly indefinite; there is additional 

 reason for inferring that organisms are products of evolution. 8 



It will be evident that these observations concerning the organic clas- 

 sification apply with equal force to the Periodic classification. For in- 

 stance, the elements of the alkaline earth family are not as sharply sepa- 

 rated from those of the alkali family as they are from the inert gases 

 or the halogens, and similar remarks apply to the other families. Within 

 each group, too, the extent to which the two families comprising it 

 differ from each other varies in the different cases. Thus, the elements 

 of the chromium family are not as sharply distinguished from those of 

 the oxygen family as the members of the copper family are from the 

 alkalies. In the case of the elements, as in that of the organisms, the 

 various groups and sub-groups differ from each other in the extent to 

 which they are distinct from corresponding groups and sub-groups ; and 

 since in the latter instance, as we have seen, this peculiarity affords an 

 additional indication of evolution, we have reason (when we consider 



6 Spencer, loc. cit., p. 361. 



' For a detailed discussion of this point, see ' ' Principles of Biology, ' ' I., 

 361-362. 



8 Spencer, loc. cit., p. 362. 



