810 ZOOLOGY scheme of classification upon the zoological world. The L. Agis- last of these was that of Louis Agassiz (Essay on Classiji- siz - cation, 1859), who, whilst surveying all previous classifica tions, propounded a scheme of his own, in which, as well as in the criticisms he applies to other systems, the leading notion is that sub-kingdoms, classes, orders, and families have a real existence, and that it is possible to ascertain and distinguish characters which are of class value, others which are only of ordinal value, and so on, so that the classes of one sub -kingdom should on paper, and in nature actually do, correspond in relative value to those of another sub -kingdom, and the orders of any one class similarly should be so taken as to be of equal value with those of another class, and have been actually so created. Influence The whole position was changed by the acquiescence, of Dar- -which became universal, in the doctrine of Darwin. That doctrine took some few years to produce its effect, but it on taxo- became evident at once to those who accepted Darwinism uomy. that the natural classification of animals, after which col lectors 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 numer ous diverging branches, the actual form of that tree, its limitation to a certain number of branches corresponding to a limited number of divergencies 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. Haeckel. The first naturalist to put into practical form the conse quences 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 Morpho- logie. 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 branchings "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, sub-orders into tribes, tribes into families, families into genera, genera into species. Additional branchings could be indicated by similar terms where neces sary. 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 naturally enough only a first attempt. In the edition of the Natiirliche Schopfunr/sgeschichte published in 1868, he made a great advance in his genealogical classi fication, since he now introduced the results of the extra ordinary activity in the study of embryology which followed on the publication of the Origin of Species. Cellular The pre-Darwinian systematists since the time of Yon olog? Baer had attached ve T great importance to embryological facts, holding that the stages in an animal s development were often more significant of its true affinities than it s adult structure. Von Baer had gained unanimous support for his dictum, " Die Entwickelungsgeschichte ist der wahre Lichttrager fiir Untersuchungen iiber organische Korper." Thus J. Miiller 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 num ber of cells, and these at last from the single egg-cell. Kb lliker (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 Kowalewsky, by his accounts of the development of Ascidians and of Amphioxw (1866), who really made zoologists see that a strict and complete cellular embryo logy 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. Kowalewsky s work appeared between the dates of the Generelle Morphologic and the Schopfungsgeschickte. 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 separa tion of the unicellular or equicellular Protozoa from the Sponges, hitherto confounded with them. It is this in troduction 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 (1873-82) of F. M. Balfour, and produced the profoundest modifications in classification. Haeckel s earlier pedigree is worth comparing with his second effort, as showing the beginning of the influence just noted. The second pedigree is as follows : Phyla. Cladcs. Protozoa. Zoophyta. Verraes. Mollusca. Echinoderma. Arthropoda. Vertebrata. OVULARIA. I BLASTULARIA. ( SPOXGI-E. j ACALEPH.E. ACCELOMI. CCELOJIATI. I ACEPHALA. EUCEPHALA. j COLOBRACHIA. LlI OBRACIIIA. ( CARIDES. 1 TRACHEATA. f ACRAXIA. MOXORRIIINA. AXAMNIA. AMNIOTA. Classes. f Archezoa. -! Grcgarinse. [_ Infusoria. ( Planseada.
Gastrseada.
Porifcra. ( Coralla. -j Hydromedusse.
Ctenophora.
Platyhclminthcs. Ncmathclminthcs Bryozoa. Tunicata. 4 Rhynchoccela. Gephyr&a. I Rotatoria. I Annelida. ( Spirobranchia.
LamcUibranchia.
( Cochlides.
Cephalopoda.
( Aster ida.
Crinoida.
( Echinida.
Holothurise.
Crustacea. ( Arachnida. -j Myrlapoda.
Insccta.
Leptocardia. Cydostoma. ( Pisces. J Dipncusta. | Halisauria. Amphibia. ( Rcptilia. -j Aves. I Mammalia, Haeckel second arrange
ment.