Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/877

Rh MORPHOLOGY 845 as another part or organ in a different animal (e.g., parachute of Draco and wings of Bird).&quot; He further distinguishes three kinds of homology : (1) special, being &quot; that above denned, namely, the correspondence of a part or organ determined by its relative position and connexions with a part or organ in a different animal, the determination of which homology indicates that such animals are constituted on a common type,&quot; e.g., basilar process of human occipital with basi-occipital of fish ; (2) general, that higher relation in which a part or series of parts stands to the fundamental or general type, involving a knowledge of the type on which the group in question is constituted,&quot; e.g., the same human bone and centrum of the last cranial vertebra ; (3) serial homology, &quot;repre sentative or repetitive relation in the segments of the same skeleton &quot; (demonstrated when general and special homologies have been determined) ; thus usually the basi-occipital and basi-sphenoid are &quot;homotypes.&quot; These terms were henceforth accepted by naturalists ; but the criterion of analogy and homology became for Agassiz and other embryologists developmental as well as comparative, reference to the ideal archetype becoming less and less frequent. Passing over the discussions of Agassiz and Hronn, of which the latter is criticized and partly incorporated by Haeckel, we find the last-named (1) placing serial under general homology ; (2) erecting categories of homology partially corre sponding to those of individuality, (a) homotypy (of antimeres), hence distinct from that of Owen, (b) homodynamy (of metameres), (c) homonomy (of parts arranged on transverse axes) ; (3) defining special homology in terms of identity of embryonic origin. In 1870 this latter point was more fully insisted upon by Ray Lan- kester, who, decomposing it into two others, proposed to supersede the term homology by homogcny, being the correspondence of common descent, and homoplasy, denoting any superinduced correspondence of position and structure in parts embryonically distinct. Thus, the fore-limb of a mammal is homogenous with that of a bird, but the right and left ventricles of the heart in both are only homoplastic, these having arisen independently since the divergence of both groups from a uni-ventriculate ancestor in relation to similarity of physiological needs. Mivart next pro posed to retain homology as a generic term, with homogeny and homoplasy as two species under it, and carried the analysis into great detail, distinguishing at first twenty-five, but later fifteen, kinds of correspondence : (1) parts similar in function only, e.g., legs of Lizard and Lobster ; (2) parts similar botli in function and relative position, wings of Bat and Bird ; (3) parts of common descent, fore-limb of Horse and Rhinoceros ; (4) parts of similar embryonic origin, whatever be their racial genetic relations, e.g., occipitals of Panther and Perch ; (5) parts of dissimilar embryonic origin, whatever be their racial genetic relations, e.g., legs of Diptera ; (6, 7, 8, 9, 10) laterally, vertically, serially, antero- posteriorly, and radially homologous parts ; (11) subordinate serial homologues, e.g., joints of antenna; (12 and 13) secondary and tertiary subordinate serial homologues ; (14 and 15) special and general homologies (in Owen s sense). In his Kalkschwammc Haeckel proposed to term homophyly the truly phylogenetic homology in opposition to homomorphy, to which genealogic basis is wanting ; and finally Von Jhering has published a repetition of Lankester s view. In this discussion, as in that of individuality, it is evident that we are dealing with numerous logical cross-divisions largely corre sponding, no doubt, to the complex web of inter-relations presented by nature, yet remaining in need of disentanglement. Though we must set aside analogies of functional activity, the resemblances in external shape or geometric ground-form which correspond to these, e.g., Hydrozoa and Bryozoa, Fishes and Cetaceans, mimetic organisms, are nevertheless, as our historic survey showed, the first which attract attention ; and these homoplastic or homomor- phic forms, as Haeckel has shown, come as fairly within the province of the promorphologist as do isomorphic crystals within that of his an-organological colleague the crystallographer. Here, too, would be considered &quot;radial,&quot; &quot;vertical,&quot; &quot;lateral&quot; homology, &quot; homotypy of antimeres,&quot; and all questions of symmetry, for which Haeckel s nomenclature of homaxonial, homopolic, &c., is distinctly preferable. Entering the field of tectology or morphology in the ordinary sense, we may next consider whether two organisms com pared are of the same category of individuality are homocategoric ; and under this serial homology, for instance, would come as a minor division, the correspondence between the units or parts of units of a linear dyad-deme or triad. From a third point of view, that of the embryologist, we trace the development of each multi- cellular organism (1) from the embryonic layers and systems into which the secondary unit (gastrula or plant embryo) differentiates, (2) from a unit-deme or unit of the inferior order or orders of individuality. The parts and units thus recognized by ontogenetic research, respectively or successively homodcrmic, homosystcmic, and homodcmic, may then conveniently be termed (indifferently save for considerations of priority) either &quot;specially homologous,&quot; &quot; homogenous,&quot; &quot; homophylic,&quot; or &quot;homogenetic,&quot; in the language of phylogenetic theory. These three great classes of morphological correspondence promorphological, tectological, and embryological may or may not coincide. But the completest homology, in which all forms of resemblance unite and from which they differentiate, is that expressed in the cell theory, or rather in that ovum theory which underlies it, and which Agassiz therefore not unjustly regarded as &quot; the greatest discovery in tho natural sciences of modern times.&quot; 1 8. Results to Taxonomy. The advance and modification of classifications which follow each morphological advance have been pointed out above, and taxonomy thus never quite reaches a level with morphological knowledge. That it requires much reform at present is obvious. Although the dogma of the constancy of species is no longer maintained, its results survive, and perhaps a majority of groups have still to be remonographed in the generalizing spirit with which Haeckel has treated the calcareous Sponges, or Car penter, Parker, and Brady the Foraminifera. The union of tho Protophyta and Protozoa into the Protista (a generalization which research is constantly confirming) involves a final abandonment of the mediaeval figment of three kingdoms of nature, and a revival of the Organisata of Linnaeus. Physiological prejudices, too, are not completely expelled ; hence, for instance, the constant attempts to separate Animalia and Vegctabilia by physiological character istics, which would be irrelevant even if in themselves valid. A strictly morphological standard must be applied to the construction of classifications and the pruning of genealogical trees ; organisms are &quot; higher &quot;or &quot; lower &quot; not according to their stage of evolution in beauty or intelligence but (as Huxley has most clearly pointed out in the essay referred to under 6) to the degree of morphological differentiation by excess, suppression, or coalescence which they exhibit. Thus the supreme position of Man in classification must be abandoned, since the Primates are simply one of the less special ized, i. e., lower orders of Mammals, and the Mammals themselves are on the whole distinctly less specialized than the Birds, or per haps even some of the higher Reptiles. . The morphological import ance of the &quot;vegetable kingdom&quot; sinks when tested by such a standard. The Cormophytes are all nothing more than an axis with appendages, and as such may fairly be compared, not with the entire animal assemblage, but merely to that group which is homomorphic (or rather isomorphic) with them as reducible to axis and appendages too. Such a group we find in the Hydrome- clusse, which we can easily model in imagination into all the special izations of the floral world, a single genus like Clava or Tubularia affording a starting-point for countless &quot;natural orders.&quot; 9. Relation of Morphology to Physiology. Although the pure morphologist investigates laws of structure only, and rightly elimi nates the conceptions of life, environment, and function, yet if kept permanently apart from physiological considerations his labours would be incomplete and his results inexplicable, if not indeed almost illusory. For, however deeply one penetrates through super ficial and adaptive characters to an apparently permanent and fundamental morphological type, this is itself but an earlier adapta tion, showing the fading traces of an earlier adaptation still. And, conversely, the most superficial of adaptive characters, if trans mitted to numerous varying descendants, may attain high morpho logical importance. The morphological aspect of an organism is merely statical, and, like that of an eddy or a vortex-ring, becomes only truly intelligible when viewed in its dynamic aspect ; and thus, though the demonstration of the structural unity of the organic world is in itself a great result, yet the desire of a deeper explanation of form as determined by function and environment is thereby rendered all the more pressing. An example may be taken from botany. Thus Airy beautifully explains the pheno mena of phyllotaxis as adaptations to bud-life. Or again, in a common flower, say the Dead-nettle, all the details of form .are in deed described by the systematist with equal minuteness (a pro ceeding which, except in so far as serving for specific identification, is of no further scientific value), but receive separate interpretation from the two distinct standpoints of the morphologist and physio logist. The latter, to whom form is important merely so far as explanatory of function, shows how the tough persistent calyx is protective against various dangers, how the corolla serves to lure the fertilizing bees, which find in its lip a landing stage and in each lateral process a hold-fast, while its hood at once protects tho pollen against rain and determines the curvature of the stamens, this curvature, as well as their didynamous arrangement, median position, and linearly arranged anther-lobes being all adaptations through the medium of the bee s hairy back to meet the similarly placed stigma of another flower, and so on. The morphologist, on the other hand, analyses the calyx into its five constituent sepals, reduces the corolla to a regular pentamerous type, ascertains the position of the four stamens, and asserts the loss of a fifth posterior one, finds the ovary to be primitively two-celled, and thus reaches a schematic conception of a not archetypal but ancestral form. This ground-form itself, however, suggests a new train of considera tions both morphological and physiological respecting the origin of 1 For bibliography see Lankester, An. Mag. Nat. Hist., 1870, or Gedtles, Jena. Zeitichr., 1SS3.