Page:Popular Science Monthly Volume 82.djvu/590

586 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 variable degree of differentiation between corresponding groups and subgroups.

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.

It will be evident that these observations concerning the organic classification apply with equal force to the Periodic classification. For instance, the elements of the alkaline earth family are not as sharply separated 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