Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/861

Rh ZOOLOGY 811 Dendri form distribu tion of animal kimrdom Iii representing pictorially the groups of the animal king dom as the branches of a tree, it becomes obvious that a distinction may be drawn, not merely between the indivi dual 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 &quot;grade&quot; 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 one 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 pro ceeding 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 genera lization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be dis tinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg- cell by &quot; cell division &quot; gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive diges tive chamber, the archenteron. Of these latter, two grades were further distinguished by Lankester, those which re main possessed of a single archenteric cavity and of two primary cell-layers (the Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or body-cavity and the metenteron or gut (Ccelomata or Triploblastica). To the primitive two -cell- layered form, the hypothetical ancestor of all 3fetazoa or Enterozoa, Haeckel gave the name Gastrxa; the embryonic form which represents in the individual growth from the egg this ancestral condition he called a &quot;gastrula.&quot; The term &quot;diblastula&quot; 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 diblas tula, the coelom, 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 Miiller, 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 prin ciple, called by Haeckel &quot;the biogenetic fundamental law,&quot; 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 inter preted by Fritz Miiller, but the authoritative view on the subject had been that there is a parallelism between (a) 1 &quot; Notes oil Embryology and Classification,&quot; in Quart. Journ. Micr. Set., 1877. the series of forms which occur in individual development, (b) 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 Miiller s law of recapitulation which gave the chief stimulus to recent cmbryological investigations ; and, though it is now recog nized that &quot; recapitulation &quot; 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 Dohru s zoological knowledge as represented by classification is the doctrine doctrine of degeneration propounded by Anton Dohrn. of 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 repre sent 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 Dohrn s merit to have pointed out : 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 duction of this conception necessarily has had a most important effect in the attempt to , unravel the gen ealogical affini- * ties of animals. It renders the task a more 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 in troduction of the new conceptions dating from Sub J r c (l 7 e d Darwin has been a healthy reaction from that attitude of mind which led to the re garding of the classes and orders recognized by Crt&amp;lt; &quot; L p R T z A authoritative zoologists Genealogical tree of animd kingdom. 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 &quot; insight &quot; of some more or less gifted persons into a plan or system which had been arbi trarily chosen by the Creator. Consequently there was a tinge of theological dogmatism about the whole matter. To deny the Linnsean, 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 Linnaean taint which is apparent 1 Ursprung der Wirbdthiere, Leipsic, 1875 ; and Laukester, Degen eration, London, 18SO. , CCE. L ENTLRA ENTEROZOA