Page:Popular Science Monthly Volume 82.djvu/589

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The fact that the groups of organisms fall naturally into a certain classification is in itself evidence of their origin by evolution. Now, the most salient characteristic of this classification is a division into groups, and a subordination of groups within groups.

If we examine the characteristics of the Periodic classification, we shall find there the same peculiarities as have been observed in zoological classifications. Thus, there are the nine groups of elements, each quite distinct from the others, and each, as we have shown, very probably having a distinct plan of atomic structure common to all the members of the group. These nine groups correspond to the twelve phyla of organisms. Each group, again, is divided into two families, corresponding to the classes into which organic phyla are divided. That we have no further subdivisions corresponding to those in the organic classification is doubtless due to the circumstance that the number of elements is extremely small as compared with the number of species of animals. When we remember that even with this small number of elements, the Periodic classification presents many irregularities—as forcing into the same family elements with widely different properties (e.g., the copper family); creating a group of "transitional elements" different in the principle of its arrangement from the other groups; the breaking of the periodic sequence by argon, which is greater in atomic weight than potassium, yet precedes it in the series, and by tellurium, which bears a similar relation to iodine; and the irregularities presented by the rare earths—when these facts are considered, it can scarcely be doubted that if the number of the elements were at all comparable to that of organic species, the classification of the elements would necessarily present a subdivision of group within group as extensive, perhaps, as that found among organisms. Moreover, the periodic relation would probably be largely obscured by the great number of its irregularities and contradictions.

Since the classification into which organisms are naturally arranged, of group subordinated to group, is regarded as an indication of evolution, as previously stated; the fact that a similar arrangement is found in the classification of the elements suggests (when we consider also