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.
. . . 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 constitute generic distinctions. So, too, it is with orders and classes; in some of which there have been introduced intermediate sub-divisions having no equivalents 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.[1]
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.[2]
If, then, the classification of organisms results in several orders of assemblages, such that assemblages of the same order are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly indefinite; there is additional reason for inferring that organisms are products of evolution.[3]
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
