Page:Popular Science Monthly Volume 82.djvu/591

Rh also the other evidence) for so regarding it in the case of the elements also.

One other analogy demands recognition. Although, as previously stated, the phyla of organisms differ widely from each other, yet animals belonging to different phyla often show marked resemblances to each other in particular features. This phenomenon is a consequence of "the identity of plan, under the most diverse conditions of organization and habits of life (which prevails) not only among animals of the same group, but also between those of different groups." For instance, regarding the affinities of the Rotifera (Phylum Trochelminthes) Parker and Haswell state that

Resemblances are also noted between the Class Gephyrea (Phylum Annulata) and Phoronis (Phylum Molluscoida). The Crustacea (Phylum Arthropoda) "belong to the same general type of organization as the articulated worms [Phylum Annulata]." Of the Phylum Mollusca it is stated that

Similarities are also pointed out between the sponges (Phylum Porifera) and the Cœlenterata.

Corresponding to these counter-resemblances in structure among organisms, we have counter-resemblances in properties among the elements. Thus, mercury (Group II.) resembles copper (Group I.) in that both form two series of compounds, monovalent and divalent respectively, both form halides insoluble in water and decomposed by light, etc. Aluminum (Group III.) is similar to chromium (Group VI.) in that the hydroxides on heating give the oxides and, respectively, in that they form no stable sulphide or carbonate, etc. Thallium (Group III.) resembles, on the one hand, lead (Group IV.) in its metallic properties, in forming a chloride with properties similar to those of lead chloride, while, on the other hand, it resembles the