The close analogies which we have shown to exist between the periodic and the zoological classifications would seem to point toward a fundamental identity of principle in these two systems. I have endeavored to show that there are in the inorganic world the exact homologues of some of the most important facts upon which the law of organic evolution rests, i. e., the evidence of the geological record and of the embryological resemblances; to emphasize the importance of the spectroscopic evidence; and to show that the Periodic classification is in its main aspects identical in its nature with the zoological classification. These facts tend to indicate that the groups of the elements correspond to the phyla of the organisms, in being the outward expression of a process of evolution. The periodicity in the arrangement of the elements is expressive of the fact that in each family there is the same plan of atomic structure, and a gradual and progressive change in this structure as we traverse the groups from the inert gases to the halogens. That it is an imperfect relationship is shown by its numerous contradictions, already mentioned. These facts, however, harmonize entirely with the evolutionary view, for zoological classifications show just such irregularities. Moreover, according to the evolutionary view, an element need not necessarily be smaller in atomic weight than the next in the same series. The evolutionary view is entirely compatible with those phenomena, which seem to be out of harmony with the Periodic classification.
If the species of organisms were few enough and their structure simple enough, it seems likely that it would be possible to select some common characteristic which would serve as a basis of periodicity corresponding to that in the elements. Conversely, as has already been indicated, if the number of the elements were at all comparable to that of organic species, it is probable that the Periodic relation would be largely obscured by the great number of its exceptions.
Without the knowledge of the fact of organic evolution, the arrangement of animals and plants into classes, with their numerous group resemblances and counter resemblances, must have seemed a purely arbitrary one, having no basis in nature. Similarly, when we consider the characters of the elements of the same families, their close resemblances to each other, and their minor resemblances to members of other families, the irregularities of the Periodic classification, etc., it is evident that we can coordinate these seemingly contradictory phenomena into a coherent whole on the basis of the evolution of the elements. The extraordinary relations disclosed by the Periodic classification are the outward and manifest signs of the process to which atoms, like organisms, owe their individual natures. The process begun in the one (the atom) continues in the other (the organism).
- "The propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classification." Darwin, quoted by Spencer, "Principles of Biology," Vol. I., p. 364.