lies (Group I.) in forming a hydroxide which is soluble in water and strongly alkaline in reaction.
Other examples, concerning which it is unnecessary to enter into detail, are the resemblances between phosphorus and sulphur; between beryllium and aluminum; between manganese and chromium; between boron, carbon and silicon; between gold and the platinum metals. It will be observed from the zoological examples above cited that the members of a phylum, while showing a greater or less similarity to each other, will often markedly resemble members of different phyla. The examples I have given show that a similar phenomenon is often characteristic of the elements of a family—the elements compared are in most cases similar to the other elements of the same family, while having at the same time the points of resemblance with each other described; and since the relationships referred to between distinct groups of organisms are believed to indicate a common origin, we may, perhaps, consider the analogous phenomena among the elements as of the same import.
Did space permit, other analogies might be pointed out between the Periodic and the zoological classifications; but enough has already been indicated to show that the Periodic classification possesses the main characteristic features of the zoological classification.[1] Now, the fact that these characteristics of the latter system are in themselves an indication of organic evolution suggests that the Periodic classification may be regarded in the same light, as I have already indicated. This suggestion is strengthened by the further evidence now to be considered.
The Homologue of the Embryological Evidence; the Phenomena of Radioactivity
The study of comparative embryology has brought to light certain facts which constitute important evidence of organic evolution; for many of the higher animals, in their immature forms, pass through stages in which they resemble more or less the adult forms of other animals, lower in the scale of differentiation. Moreover, animals of distinct but related species, in the progress of their development, often show marked similarities of structure. Von Baer
found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structure displayed at corresponding phases by a less extensive multitude of organisms; that at each subsequent stage, traits are acquired which successively distinguish the developing embryo from groups of embryos that it previously resembled—thus step by step diminishing the class of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member.[2]
