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."[1] For instance, regarding the affinities of the Rotifera (Phylum Trochelminthes) Parker and Haswell[2] state that
Their general resemblance to the free-swimming larvae of Annelids is extremely close. . . . The excretory organs recall those of Platyhelminthes, and also resemble the provisional nephridia or head-kidneys of Annulate larvæ.
Resemblances are also noted between the Class Gephyrea (Phylum Annulata) and Phoronis (Phylum Molluscoida).[3] The Crustacea (Phylum Arthropoda) "belong to the same general type of organization as the articulated worms [Phylum Annulata]."[4] Of the Phylum Mollusca it is stated that
The Mollusca. . . form an extremely well defined phylum, none of the adult members of which approach the lower groups of animals in any marked degree. There are, however, clear indications of affinity with "worms," especially in the frequent occurrence of a trochosphere stage in development, in the presence of nephridia, and in the occurrence, in Amphineura and some of the lower Gastropods, of a ladder-like nervous system resembling that of some Turbellaria and of the most worm-like of Arthropods, Peripatus. Rhodope, moreover, shows certain affinities with flat worms.[5]
Similarities are also pointed out between the sponges (Phylum Porifera) and the Cœlenterata.[6]
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 alka-
- ↑ Claus and Sedgwick, "Zoology," Vol. I., p. 54.
- ↑ "Zoology," Vol. I., pp. 309-310.
- ↑ Parker and Haswell, loc. cit., p. 461. The zoological classification followed throughout is that given by these authors.
- ↑ Parker and Haswell, loc. cit., p. 556.
- ↑ Parker and Haswell, loc. cit., pp. 750-751.
- ↑ Parker and Haswell, loc. cit., pp. 215-216.
