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 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 Kovalevsky’s work led to a generalization, independently arrived 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 Metazoa 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 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 (Coelomata or Triploblastica). To the primitive two-cell-layered form, the hypothetical ancestor of all Metazoa or Enterozoa, Haeckel gave the name Gastraea; the embryonic form which represents in the individual growth from the egg this ancestral condition he called a “gastrula.” The term “diblastula” was subsequently adopted in England for the gastrula of Haeckel. The tracing of the exact mode of development, cell by cell, of the diblastula, the coelom, and the various tissues of examples of all classes of animals was in later years pursued with immense activity and increasing instrumental facilities.
Two names in connexion with post-Darwinian taxonomy and the ideas connected with it require brief mention here. Fritz Müller, by his studies on Crustacea (Für Darwin, 1864), showed the way in which Fritz Müller’s recapitu-
lation.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 Müller, 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, (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 Müller’s law of recapitulation which gave the chief stimulus to embryological investigations between 1865 and 1890; 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 conception of branches. Dohrn’s doctrine
of degen-
eration.The first post-Darwinian systematists naturally and without reflexion accepted the idea that existing simpler forms represent stages 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[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 in other cases continuous maintenance of the status quo. The introduction of this conception necessarily has had a most important effect in the attempt to unravel the genealogical affinities 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 introduction of the new conceptions dating from Darwin was 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 some 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 Linnaean, or later the Cuvierian, classes was very much like denying the Mosaic cosmogony. But systematic zoology is now entirely free from any such prejudices, and the Linnaean taint which is apparent even in Haeckel and Gegenbaur may be considered as finally expunged.
Sub-Grade B. CŒLOMATA.
Sub-Grade A. CŒLENTERA.
Grade 2. ENTEROZOA.
Grade 1. PROTOZOA.
A genealogical tree of animal kingdom (Lankester, 1884).
There are, and probably always will be, differences of opinion as to the exact way in which the various kinds of animals may be divided into groups and those groups arranged in such an order as will best exhibit Lankester’s system.their probable genetic relationships. The main divisions which, writing in 1910, the present writer prefers, are those adopted in his Treatise on Zoology (Part II. ch. ii.) except that Phylum 17, Diplochorda (a name doubtfully applicable to Phoronis) is replaced by Podaxonia, a term employed by Lankester in the 9th edition of this-encyclopedia and now used to include a number of groups of doubtful but possible affinity. The terms used for indicating groups are “Phylum” for the large diverging branches of the genealogical tree as introduced by Haeckel, each Phylum bears secondary branches which are termed “classes,” classes again branch or divide into orders, orders into families, families into genera, genera into species. The general purpose is to give something like an equivalence of importance to divisions or branches indicated by the same term, but it is not intended to imply that every phylum has the
- ↑ Ursprung der Wirbelthiere (Leipzig, 1875); and Lankester, Degeneration (London, 1880).
