Page:EB1911 - Volume 28.djvu/1058

Rh value with those of another class, and have been actually so created.

The whole position was changed by the acquiescence, which became universal, in the doctrine of Darwin. That doctrine took some few years to produce its effect, but it became evident at once to those who accepted Darwinism that the natural classification of animals, after which collectors and anatomists, morphologists, philosophers and embryologists had been so long striving, was nothing more nor less than a genealogical tree, with breaks and gaps of various extent in its record. The facts of the relationships of animals to one another, which had been treated as the outcome of an inscrutable law by most zoologists and glibly explained by the transcendental morphologists, were amongst the most powerful arguments in support of Darwin’s theory, since they, together with all other vital phenomena, received a sufficient explanation through it. It is to be noted that, whilst the zoological system took the form of a genealogical tree, with main stem and numerous diverging branches, the actual form of that tree, its limitation to a certain number of branches corresponding to a limited number of divergences in structure, came to be regarded as the necessary consequence of the operation of the physico-chemical laws of the universe, and it was recognized that the ultimate explanation of that limitation is to be found only in the constitution of matter itself.

The first naturalist to put into practical form the consequences of the new theory, in so far as it affected zoological classification, was Ernst Haeckel of Jena (b. 1834), who in 1866, seven years after the publication of Darwin’s Origin of Species, published his suggestive Generelle Morphologie. Haeckel introduced into classification a number of terms intended to indicate the branchings of a genealogical tree. The whole “system” or scheme of classification was termed a genealogical tree (Stammbaum); the main branches were termed “phyla,” their branching’s “sub-phyla”; the great branches of the sub-phyla were termed “cladi,” and the “cladi” divided into “classes,” these into sub-classes, these into legions, legions into orders, orders into sub-orders, suborders into tribes, tribes into families, families into genera, genera into species. Additional branchings could be indicated by similar terms where necessary. There was no attempt in Haeckel’s use of these terms to make them exactly or more than approximately equal in significance; such attempts were clearly futile and unimportant where the purpose was the exhibition of lines of descent, and where no natural equality of groups was to be expected ex hypothesi. Haeckel’s classification of 1866 was only a first attempt. In the edition of the Natürliche Schöpfungsgeschichte published in 1868 he made a great advance in his genealogical classification, since he now introduced the results of the extraordinary activity in the study of embryology which followed on the publication of the Origin of Species.

The pre-Darwinian systematists since the time of Von Baer had attached very great importance to embryological facts, holding that the stages in an animal’s development were often more significant of its true affinities than its adult structure Von Baer had gained unanimous support for his dictum, “Die EntwickelungsgescTiichte ist der wahre Lichttrager für Untersuchungen iiber organische Körper.” Thus J. Müller’s studies on the larval forms of Echinoderms and the discoveries of Vaughan Thompson were appreciated. But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell. Kölliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier. Kovalevsky’s work appeared between the dates of the Generelle Morpholgie and the

Schöpfungsgcschihite. Haeckel himself, with his pupil Miklucho-Maclay, had in the meantime made studies on the growth from the egg of Sponges— studies which resulted in the complete separation of the unicellular or equicellular Protozoa from the Sponges, hitherto confounded with them. It is this introduction of the consideration of cell-structure and cell-development which, subsequently to the establishment of Darwinism, has most profoundly modified the views of systematists, and led in conjunction with the genealogical doctrine to the greatest activity in research—an activity which culminated in the work (1873–1882) of F. M. Balfour, and produced the profoundest modifications in classification.

Haeckel’s second pedigree is as follows:—

In representing pictorially the groups of the animal kingdom as the branches of a tree, it becomes obvious that a distinction may be drawn, not merely between the individual main branches, but further as to the level at which they are given off from the main stem, so that one branch or set of branches may be marked off as belonging to an earlier or lower level than another set of branches; and the same plan may be adopted with regard to the clades, classes and smaller branches. The term “grade” was introduced by Ray Lankester (“Notes, on Embryology and Classification,” in Quart. Journ. Micr. Sci. 1877), to indicate this giving off of branches at a higher or lower, i.e. a later or earlier, level of a main stem. The mechanism for the statement of the genealogical relationships of the groups of the animal kingdom was thus completed. Renewed study of every group was the result of the acceptance of the genealogical idea and of the recognition of the importance