The Principles of Biology, Volume 1 (of 2)

by Herbert Spencer

§ 100a. Two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. Replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. Concerning the present attitude of zoologists, Prof. MacBride writes:—

"They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other.

"The phyla recognized at present are:—

(1) Protozoa.
(2) Porifera (Sponges).
(3) Cœlenterata.
(4) Echinodermata.
(5) Platyhelminthes		Cestodes. Trematodes. Turbellaria.
(6) Nemertea.
(7) Nematoda.
(8) Acanthocephala (Echinorhyncus).
(9) Chætognatha (Sagitta).
(10) Rotifera.
(11) Annelida (Includes Leeches and Gephyrea, Chætifera).
(12) Gephyrea, Achæta.
(13) Arthropods		Tracheata (Peripatus, Myriapods, Insects). Arachnids. Crustacea. Pycnogonida.
(14) Mollusca.
(15) Polyzoa (Including Phoronis).
(16) Brachiopoda.
(17) Chordata (Includes Balanoglossus and Tunicates. Some continental zoologists do not admit Balanoglossus)."

[This last phylum of course includes the Vertebrata.]

Though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. On the adjacent page I have tried to make a tentative arrangement of this kind.

At the bottom of the table I have placed together, under the name "Compound Protozoa," those kinds of aggregated Protozoa which show no differentiations among the members of groups, and are thus distinguished from Metazoa; and I have further marked the distinction by their position, which implies that from them no evolution of higher types has taken place. Respecting the naming of the sub-kingdoms, phyla, classes, orders, &c., I have not maintained entire consistency. The relative values of groups cannot be typographically expressed in a small space with a limited variety of letters. The sizes of the letters mark the classificatory ranks, and by the thickness I have rudely indicated their zoological importance. In fixing the order of subordination of groups I have been aided by the table of contents prefixed to Mr. Adam Sedgwick's Student's Text Book of Zoology and have also made use of Prof. Ray Lankester's classifications of several sub-kingdoms.

Let me again emphasize the fact that the relationships of these diverging and re-diverging groups cannot be expressed on a flat surface. If we imagine a laurel-bush to be squashed flat by a horizontal plane descending upon it, we shall see that sundry of the upper branches and twigs which were previously close together will become remote, and that the relative positions of parts can remain partially true only with the minor branches. The reader must therefore expect to find some of the zoological divisions which in the order of nature are near one another, shown in the table as quite distant.

§ 101. While the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. When aggregating the smallest groups into larger groups and these into groups still larger, they have adopted certain general terms expressive of the successively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in Nature. It has been taken for granted that species, genera, orders, and classes, are assemblages of definite values—that every genus is the equivalent of every other genus in respect of its degree of distinctness; and that orders are separated by lines of demarcation which are as broad in one place as another. Though this conviction is not a formulated one, the disputes continually occurring among naturalists on the questions, whether such and such organisms are specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where not avowed. Yet that differences of opinion like these arise and remain unsettled, except when they end in the establishment of sub-species, sub-genera, sub-orders, and sub-classes, sufficiently shows that the conviction is ill-based. And this is equally shown by the impossibility of obtaining any definition of the degree of difference which warrants each further elevation in the hierarchy of classes.