MYRIAPODA. Orders : Chi/og-natha and Chi/opo(las ARACIINIDs. Orders : Pub/tow/rift and Trachearia.
Class IV. CRUSTACEA.
Section 1. Podophatalmia.
Orders : Dee( T00% and Stomopodm Section 2. Bdriophthalmi.
Orders: Amphipoda, Lcemodipoda, and isopoda.
Section 3. Branchiopoda.
Orders : Ostraeoda, Phyllopoda, and Tidobate.
Section 4. Entomostraea.
Orders : Copepod«, Cladoccra, Siphonostoma, Lernxida, Cirripedia.
Section 5. Xiphosura.
(The orders of the classes which follow are not given in the work quoted.) TUI:BELLARIA.
Branch III. Malacozoaria or Sub-Branch 1. Mollusca pro Class I. CEPHALOPODA.
Sub-Branch 2. Molluscoidea Class I. TUNICATA.
Branch IV. Zoophytes. Sub-Branch 1. Radiaria.
Class I. ECHINODERMS.
Class I. INFUSOR1A.
In England T. H. Huxley adopted in his lectures (1869) a classification which was in many respects similar to both of the foregoing, but embodied improvements of his own. It is as follows :- Sub-Kingdom I. Protozoa.
Classes : RHIZOPODA, GREGARINIDA, RADIOLAR1A, SPONGIDA. Sub-Kingdom II. Infusoria.
Sub-Kingdom III. Ccelenterata.
Classes : I-IYDROZOA, ACTINOZOA.
Sub-Kingdom IV. Annuloida.
Classes : SCOLECIDA, ECIIINODERMATA.
Sub-Kingdom V. Annulosa.
Classes: CRUSTACEA, ARACHNIDA, MYRIAPODA, INSECTA, CILETOGNATHA, ANYELIDA.
Sub-Kingdom VI. Molluscoida.
Classes : POLYZOA, I3RACH/OPODA, TUNICATA.
Sub-Kingdom VII. Mollusca.
Classes: LAMELLIBRANCHIATA, BRANCHIOGASTROPODA, PULMOGASTROPODA, PTEROPODA, CEPHALOPODA.
Sub-Kingdom VIII. Vertebrata.
Classes : PISCES, AMPHIBIA, REPTILIA, AYES, MAMMALIA.
We now arrive at the period when the doctrine of organic evolution was established by Darwin, and when naturalists, being convinced by him as they had not been by the transmutationists of fifty years' earlier date, were compelled to take an entirely new view of the significance of all attempts at framing a " natural " classification.
Many zoologists - prominent among them in Great ClassifiBritain being Huxley - had been repelled by the airy fancies catimis and assumptions of the "philosophical " morphologists. bstarsueed. on The efforts of the best minds in zoology had been directed t„„. for thirty years or more to ascertaining with increased accuracy and minuteness the structure, microscopic and gross, of all possible forms of animals, and not only of the adult structure but of the steps of development of that structure in the growth of each kind of organism from the egg to maturity. Putting aside fantastic theories, these observers endeavoured to give in their classifications a strictly objective representation of the facts of animal structure and of the structural relationships of animals to one another capable of demonstration. The groups within groups adopted for this purpose were necessarily wanting in symmetry : the whole system presented a strangely irregular character. From time to time efforts were made by those who believed that the Creator must have followed a symmetrical system in his production of animals to force one or other artificial, neatly balanced Class IV. CESTOIDEA.
„ V. ROTATORIA.
Class II. BRYOZOA.
Class III. COP,ALLARIA OT Class II. SPONGIARIA.
Huxley's classi ti -cation.
scheme of classification upon the zoological world. The last of these was that of Louis Agassiz (Essay on Classification, 1859), who, whilst surveying all previous classifications, 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.
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 stein and numerous 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.
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 Cenerelle 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 branchings " subphyla" ; 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 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 naturally enough only a first attempt. In the edition of the Natibliche ,S'chonfungsgeschicide 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 Entwickelungsgeschichte ist der •ahre Lichttriiger ffir Untcrsuchungen iiber organische 'Carper." Thus J. Muller'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. Kiilliker (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 Asciclians and of Amphioxvs (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. Kowalewsky's work appeared between the dates of the G'enerelle Morphologic and the SchOpfungsgeschichte. 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 (1873-82) 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 : - 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 Lankester 1 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 of cellular embryology. On the On e hand, the true method of arriving at a knowledge of the genealogical tree was recognized as lying chiefly in attacking the problem of the genealogical relationships of the smallest twigs of the tree, and proceeding from them to the larger branches. Special studies of small families or orders of animals with this object in view were taken in hand by many zoologists. On the other hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kowalewsky's work led to a generalization, independentlyarrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the ..ifetazoa or Enterozoa, in which the egg-cell by "cell division " gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron. Of these latter, two grades were further distinguished by Lankester, - those which remain possessed of a single archenteric cavity and of two primary cell-layers (the Ctelenterct or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the ccelom or body-cavity and the metenteron or gut (Ccelontata or Triploblastica). To the primitive two-cell-layered form, the hypothetical ancestor of all ifetazoa or Enterozoa, Haeckel gave the name Gastr,Ta; the embryonic form which represents in the individual growth from the egg this ancestral condition he called a "gastrula." The term " diblastula " has more recently been adopted in England for the gastrula of Haeckel. The tracing of the exact mode of development, cell by cell, of the diblastula, the cmlom, and the various tissues of examples of all classes of animals has been pursued during the last twenty years with immense activity and increasing instrumental facilities, and is still in progress.
Two names in connexion with post-Darwinian taxonomy and the ideas connected with it require brief mention here. Fritz Muller, by his studies on Crustacea (Fur Darwin, 1864), showed the way in which genealogical theory may be applied to the minute study of a limited group. He is also responsible for the formulation of an important principle, called by Haeckel "the biogenetic fundamental law," viz., that an animal in its growth from the egg to the adult condition tends to pass through a series of stages which are recapitulative of the stages through which its ancestry has passed in the historical development of the species from a primitive form ; or, more shortly, that the development of the individual (ontogeny) is an epitome of the development of the race (phylogeny). Pre-Darwinian zoologists had been aware of the class of facts thus interpreted by Fritz Miller, but the authoritative view on the subject had been that there is a parallelism between (a) the series of forms which occur in individual development, (6) the series of existing forms from lower to higher, and (c) the series of forms which succeed one another in the strata of the earth's crust, whilst an explanation of this parallelism was either not attempted, or was illusively offered in the shape of a doctrine of harmony of plan in creation. It was the application of Fritz Miller's law of recapitulation which gave the chief stimulus to recent embryological investigations ; and, though it is now recognized that " recapitulation " is vastly and bewilderingly modified by special adaptations in every case, yet the principle has served, and still serves, as a guide of great value.
Another important factor in the present condition of zoological knowledge as represented by classification is the doctrine of degeneration propounded by Anton Dohrn. Lamarck believed in a single progressive series of forms, whilst Cuvier introduced the conception of branches. The first post-Darwinian systematists naturally and without re-flexion accepted the idea that existing simpler forms represent stages in the gradual progress of development, - are in fact survivors from past ages which have retained the exact grade of development which their ancestors had reached in past ages. The assumption made was that (with the rare exception of parasites) all the change of structure through which the successive generations of animals have passed has been one of progressive elaboration. It is 1)ohrn's merit to have pointed out 1 that this assumption is not warranted, and that degeneration or progressive simplification of structure may have, and in many lines certainly has, taken place, as well as progressive elaboration and continuous maintenance of the status quo. The intro complicated one ; at the same time it removes some serious difficulties and throws a flood of light on every group of the animal kingdom.
One result of the introduction of the new conceptions dating from Darwin has been a healthy reaction from that attitude of mind which led to the regarding of the classes and orders recognized by authoritative zoologists - as sacred institutions which were beyond the criticism of ordinary men. That state of mind was due to the fact that the groupings so recognized did not profess to be simply the result of scientific reasoning, but were necessarily regarded as the expressions of the " insight " of sonic more or less gifted persons into a plan or system which had been arbitrarily chosen by the Creator. Consequently there was a tinge of theological dogmatism about the whole matter. To deny the Linninan, or later the Cuvierian, classes was very much like denying the Mosaic cosmogony. At the present time systematic zoology is entirely free from any such prejudices, and the Linntean taint which is apparent even in Haeckel and Gegenbaur may be considered as finally expunged.
We give below the classification of Ray Lankester as an example of the most recent genealogical classification. It is represented by the above genealogical tree and the tabular statement which follows. The chief points in this classification are the inclusion of Belaitoglossus and the Tunicata in the phylum Vertebrate, the association of the Rotifera and the Cinetopocla with the Art7rropocla in the phylum Appendiculata, the inclusion of Lintuitts and the Earypterina in the class Arachnida, and the total abandoning of the indefinite and indefensible group of "Venues."
Grade A. PLASTIDOZOA (PROTOZOA).
Grade a. Gl'illN0.31-1'XA.
Class I. PROTEOMORPHA (no orders recognized).
Orders.-1. Sorophora. 2. Endosporea. 3. Exosporea.
Orders.-1. Nada. 2. Testaeca, IV. LABYIUNTIIULIDEA (no orders).
Orders. -1. Aphrothoraca. 2. Chiamydophora. 3.
Chalarothoraca. 4. Desmothoraca.
Sub-Class a. Imperforata.
Orders.-1. Gromiidea. 2. Astrorltizitlea. 3. IIIilioUtica. 4. Littiolidect.
Sub-Class b. Perforate.
Orders.-1. Textularidea. 2. Chilostomellidea. 3. Lagenitica. 4. Globigcriaidea. 5. Rotalidea. 6. NuIrtmulinidea.
Sub-Class a. Silicosheleta.
Orders.-1, Periivixa. 2. illonopylsect. 3. Phxodaria. Sub-Class b. Acanthinoslccleta.
Grade b. CORTICA TA.
Class I. SPOROZOA.
Sub-Class a. Gregarinidea.
Orders.-1. Haplocyta. 2. Septata.
Sub-Class b. Coccicliidect.
Orders.-1. Monosporea. 2. Oligosporea. 3. Polysporea.
Sub-Class e. illyxosporidea (no orders).
„ d. Sarcoeystidect (no orders).
Sub-Class a. Lissollagellata.
Orders.-1. 11fonarliclea. 2. Euglenoidea. 3. Eder°. mastigoda. 4. Isomastigoda.
Sub-Class b. Choanofiagedlata.
Orders.-1. Nuda. 2. Loricata. 3. Celatinigera.
Orders.-1. Actinida. 2. Dinifera.
IV. 11.11YNCII0FLAGELLATA (no orders).
Orders.-1. Peritricha. 2. Hetcrotrielta. 3. Ho/otricha. 4. Ilypotrielict.
Orders.-1. Suctoria. 2. Non-Suctoria.
Grade B. ENTEROZOA.
Sub-Grade A. CU'LENTERA.
Phylum 1. NEMATOPHORA.
Class I. HYDROMEDUS.E.
Orders.-1. Anthomedusx (Cymnoblastan). 2. Leptontedusx (Calyptoblastoza). 3. Narcomedusx. 4. Trachonteclusx. 5. Hyclroeorallinx. 6. Siphon«. phora.
„ II. SCYPHOMEDITS.E.
Sub-Class a. Tetrameralia.
Orders.-1. Calyeozoa. 2. Peromedusx. 3. Cybotnedusx. Sub-Class b. Oetomeralia.
Orders.-1. Cannostomx. 2. SCMOSIOMEE. 3. Rhizostonim. ANTIIOZOA.
Sub-Class a. Actiniamorpha.
Orders.-1. Aetiniari«. 2. Antipatharia. 3. Madreporaria.
Sub-Class b. AleyonlomtnTha.
Orders.-1. Protaleyonaria. 2. Stolonifera. 3. Aleyonaria. 4. Paragorgonaria. 5. Gorgonaria. 6. Pennat2Ilaria. 7. fielioporaria.
Class IV. CTENOPIIORA.
Orders.-1. Saccata. 2. Ettrystoma. 3. Teniata. 4. Lobata.
Phylum 2. PORIFERA.
Class I. CALcisroNGLE (= Hegamastietor«).
Orders.-1. Ilomoecela. 2. Heterocxl«.
„ II. SiLicoSPONGLE (=illieromastictora).
Sub-Class 1. Ifyalospongix. Order. Ilexaetinellaria.
Sub-Class 2. Demospongix.
Orders.-1. Tetractincllaria. 2. atronaxonaria. 3. Ealisarcaria.
Sub-Grade B. C(ELOJIA TA.
Phylum 1. VERTEBRATA.
Branch A. Hemiehorda (Balanoglossns).
„ B. Urochorda.
Grade I. Larvalia (no divisions recognized).
„ II. Saccata.
Class I. ASGIDIZE.
Orders.-1. Simplices. 2. Sociales. 3. Composite:. 4. Pyrosomlidea.
„ II. SALPIFORMIA.
Orders.-1. Salpildea. 2. Doliolidea.
Branch C. Cephalochorda (Am/Al:or-as).
Grade I. Cyclostoma.
Class I. MYNINOIDEA (no orders).
„ II. PETBOMYZONTIA (no orders).
Grade II. Gnathostoma.
Sub-Grade a. Branchiata Heterodactyla.
Class I. PISCES.
Sub-Class I. Selachii.
Orders. -1. Squall. 2. Rail.
Sub-Class 2. Holocephali (no orders).
, 3. Cannitici..
Orders.-I. Chondrostei. 2. Polypterini. 3. Lepidosteini. 4. Amidini:. 5. Cephalaspidini. 6. Placodermi. 7. Acanthoclini. 8. Pycnodontini. 9. Ccelaeanthini. 10. Dipterini.
Sub-Class 4. Telcostei.
Orders.-1. Physostomi. 2. Pharyngognathi. 8. Anacanthini. 4. Acanthopteri. 5. Plectognathi. 6. Lophobranchia.
„ II. DIPN01.
Orders.-1. Monopncumottes. 2. Dip)101971022CS.
Sub-Grade b. Branchiata Pentadactyla.
Class I. AMPIIIBIA.
Sub-Class 1. Urodela.
Orders.-1. Tchthyoidea. 2. Salamandrina. Sub-Class 2. Cymnophiona (Cmcil4a, &c.).
„ 3. Stegoeepleala (Labyrinthotion, &c.).
„ 4. Anura (Rana, &c.).
Sub-Grade e. Lipobranchia Pentadactyla. Branch a. MONOCONDYLA.
Class I. IlEITILIA.
Sub-Class 1. Chelonia (orders not tabulated). 2. Laeertilia.
Orders.-1. Rhyneltoccphala. 2. Chamxleonina. 3. Laceranct.
Sub-Class 3. Ophidia.
Orders.-1. Opoterodonta. 2. Colubriformia. 3. Proteroglypita. 4. Solenoglypha.
Sub-Class 4. Crocodilia.
Orders.-1. Ampicietelia. 2. Opisthoecelia. 3. Proco3lia.
Sub-Class 5. Pterosaltria (orders not tabulated). , 6. Dinosauria.
Orders.-1. Savropoda. 2. Stegosauria. 3. Ornithopoda. 4. Theriopoda. 5. Ccelitria. 6. Compsog7zatka. 7. Ifallopotia.
Sub-Class 7. Anomodontia (Dicynodon, &c. ).
„ 8. Plesiosauria.
„ 9. Iethyosauria.
„ II. AYES.
Grade a. Phanerodactyla.
Grade b. Cryptodactyla.
Orders. -1. Ratitce. 2. Carinatx.
Branch b. A MPH ICONDYLA Class. MAMMALIA.
Grade 1. Monotrema (Ornithorhyneltus and Echidna).
Grade 2. Ditrema.
Branch a. Marscepialia.
Orders.-1. B«rypoda. 2. Mite/Aar. 3. Jlacropoda. 4. Carpal)lulga. 5. Palimana. 6. Cantharophaga. 7. Crcophaga. 8. Edoitzda.
Branch b. Placentalia.
Sub-Branch a. Typidentata.
Orders.-1. Proinsectivora. 2. Insectivora. 3. Carnirora. 4. Cceacca. 5. Ungulate. 6. Amblypoda. 7. Sircnia. 8. Toxodontia. 9. Rodentia. 10. Proboscidca, 11. Hyracoidea. 12. Chciroptera. 13. ProsiDtix. 14. Simix.
Sub-Branch b. Edentata.
Orders.-1. Bradypoda. 2. Effodientia.
Phylum 2. ECHINODERMA. Branch A. Ambulacrata.
Class I. HOLOTHURIDEA.
Orders.-1. Elasipoda. 2. Pcdata. 3. Apoda.
EcnixoinEA. Grade a. Palmechini (Mclonites, Eocddaris).
Grade b. Autechini. Branch 1. Dcsmosticha.
Orders.-1. Regcaaria. 2. Erocyclica.
Branch 2. Petalosticha.
Orders.-1. Clypeast•ina. 2. Spatangina.