MORPHOLOGY, (Gr. μορφή, form), a term introduced by Goethe to denote in biology the study of the unity of type in organic form (for which the Linnaean term “Metamorphosis” had formerly been employed). It now usually covers the entire science of organic form. There are numerous restricted senses of the term in various sciences, but here we shall deal with it as a substantive side of zoology and botany.
Historical Outline.—If we disregard such vague likenesses as those expressed in the popular classifications of plants by size into herbs, shrubs and trees, or of terrestrial animals by habit into beasts and creeping things, the history of morphology begins with Aristotle. Founder of comparative anatomy and taxonomy, he established eight great divisions (to which are appended certain minor groups)—Viviparous Quadrupeds, Birds, Oviparous Quadrupeds and Apoda, Fishes, Malakia, Malacostraca, Entoma, and Ostracodermata—distinguishing the first four groups as Enaima (“with blood”) from the remaining four as Anaima (“bloodless”). In these two divisions we recognize the Vertebrata and Invertebrata of J. B. P. A. Lamarck, the first four groups corresponding with the Mammals, Birds, Reptiles, Fishes, whilst the others agree more loosely with the Cephalopods, Crustacea, Insecta, and Echinoderms with Mollusca other than the Cephalopods. Far from committing the mistake attributed to him of reckoning Bats as Birds, or Cetaceans as Fishes, he discerned the true affinities of both, and erected the latter into a special γένος beside the Viviparous Quadrupeds, more on account of their absence of limbs than of their aquatic habit. Not only is his method inductive, and his groups founded on the aggregate of known characters, but he foreshadows such generalizations as those of the correlation of organs, and of the progress of development from a general to a special form afterwards established by G. L. Cuvier and K. E. von Baer respectively. In the correspondence he suggests between the scales of Fishes and the feathers of Birds, or in that hinted at between the fins of Fishes and the limbs of Quadrupeds, the idea of homology is nascent; and from the compilation of his disciple Nicolaus of Damascus, who regards leaves as imperfectly developed fruits, he seems almost to have anticipated the idea of the metamorphosis of plants. Even after the reappearance of Aristotle’s works in the 13th century, little can be recorded but revivals of his conclusions. Monographs on groups of plants and animals frequently appeared, those of P. Belon on Birds and G. Rondelet on Fishes being among the earliest; and in the former of these (1555) we find a comparison of the skeletons of Bird and Man in the same posture and as nearly as possible bone for bone—an idea which, despite the contemporaneous renaissance of human anatomy initiated by Vesalius, disappeared for centuries, unappreciated save by the surgeon Ambroise Paré. B. Palissy, like Leonardo da Vinci before him, discerned the true nature of fossils; and such flashes of insight continued to appear from time to time during the 17th century. Thus, Joachim Jung recognized “the distinction between root and stem, the difference between leaves and foliaceous branches, the transition from the ordinary leaves to the folia floris,” and W. Harvey anticipated the generalizations of modern embryology by his researches on development and his theory of epigenesis.
The encyclopaedic period of which Gesner is the highest representative was continued by Aldrovandi and others in the 17th century; but, aided by the Baconian movement, then influencing all scientific minds, it developed into one of genuinely systematic aim. At this stage of progress the most important part was taken by John Ray, whose classificatory labours among plants and animals were crowned with success. He first expelled the fabulous monsters and prodigies of which the encyclopaedists had handed on the tradition from medieval times, and succeeded, particularly among plants, in distinguishing many natural groups, for which his own terms sometimes survive—e.g. Dicotyledons and Monocotyledons, Umbelliferae and Leguminosae. The true precursor of Linnaeus, he introduced the idea of species in natural history, and reformed the practice of definition and terminology. Of the works which followed up Ray’s systematic labours, none can be even named until we come to those of his great successor Linnaeus, whose grasp of logical method and lucidity of thought and expression enabled him to reform and reorganize the whole labours of his predecessors into a compact and definite “systema naturae.” The very genius of order, he established modern taxonomy, not only by the introduction of the binomial nomenclature and the renovation of descriptive terminology and method, but by the subordination of the species under the successive higher categories of genus, order and class, so reconciling the analytic and synthetic tendencies of his predecessors. Although the classification of plants by the number of their essential organs is highly artificial, it must be remembered that this artificiality is after all only a question of degree, and that he not only distinctly recognized its provisional character but collected and extended those fragments of the natural system with which A. de Jussieu soon afterwards began to build. His classification of animals, too, was largely natural, and, though on the whole he lent his authority to maintain the notion of three kingdoms of nature, he at least at one time discerned the fundamental unity of animals and vegetables, and united them in opposition to the non-living world as Organisata. At the same time he was still far more a scholastic naturalist than a modern investigator.
While the artificial system was at the zenith of its usefulness, Bernard de Jussieu was arranging his gardens on the lines afforded by the fragmentary natural system of Linnaeus. His ideas were elaborated by his nephew Antoine de Jussieu, who published diagnoses of the natural orders, so giving the system its modern character. Its subsequent elaboration and definite establishment are due mainly to the labours of Pyrame de Candolle and Robert Brown. The former concentrated his own long life and that of his son upon a new “systema naturae,” the colossal Prodromus systematis naturalis (20 vols., 1818–1873), in which 80,000 species were described and arranged. Meanwhile the penetrative genius of Brown enabled him to unravel such structural complexities as those of Conifers and Cycads, Orchids and Proteaceae, thus demonstrating the possibility of ascertaining the systematic position of even the most highly modified floral types. Both Candolle and Brown were thus no mere systematists, but genuine morphologists of the modern school.
The labours of Bernard and Antoine de Jussieu initiated a parallel advance in zoology, the joint memoir on the classification of mammals with which Cuvier and Geoffroy St-Hilaire almost began their career receiving its dominant impulse from the “genera” of Antoine. Cuvier’s works correspond in zoology to those of the whole period from the Jussieus to Brown, and epitomize the results of that line of advance. Although in some respects preceded by A. von Haller and J. Hunter, who compared, though mainly with physiological aim, the same parts in different organisms, and much more distinctly by Vicq d’ Azyr, the only real comparative anatomist of the 18th century, he opens the era of detailed anatomical research united with exact comparison and clear generalization. The Règne animal (1817) and the theory of types (vertebrate, molluscan, articulate, and radiate) are the results of this union of analysis and synthesis and mark the reconstitution of taxonomy on a new basis, henceforth to be no longer a matter of superficial description and nomenclature but a complete expression of structural resemblances and differences. In Germany, L. H. Bojanus, J. F. Meckel, C. T. E. von Siebold and Johannes Müller, with his many pupils, carried on the work; in France, too, a succession of brilliant anatomists, such as A. De Quatrefages, A. Milne-Edwards and H. de Lacaze-Duthiers, were his intellectual heirs; and in England he has been admirably represented by Sir R. Owen.
It is now necessary to return to Linnaeus, whose more speculative writings contain, though encumbered by fantastic hypotheses, the idea of floral metamorphosis. About the same time, and quite independently, C. F. Wolff, the embryologist, stated the same theory with greater clearness, for the first time distinctly reducing the plant to an axis bearing appendages—the vegetative leaves—which become metamorphosed into bud-scales or floral parts through diminution of vegetative force. Thirty years later the same view was again independently developed by Goethe in his now well-known pamphlet (Versuch die Metamorphose der Pflanzen zu erklären, Gotha, 1790). In this brilliant essay the doctrine of the fundamental unity of floral and foliar parts is clearly enunciated, and supported by arguments from anatomy, development and teratology. All the organs of a plant are thus modifications of one fundamental organ—the leaf—and all plants are in like manner to be viewed as modifications of a common type—the Urpflanze. Whether, as some historians hold, his “Urpflanze” was a mere ideal archetype, bringing forth as its fruit the innumerable metaphysical abstractions of the Naturphilosophie, and leading his countrymen into all the extravagances of that system; or whether, as E. H. Haeckel maintains, it represented a concrete ancestral form, so anticipating the view of modern evolutionists, it is certain that to him F. W. S. von Schelling was indebted for the foundation upon which he erected his philosophic edifice, as also that Goethe shared the same ideas. It must be remembered that he lived and made progress for forty years after the publication of this essay, that he was familiar with the whole scientific movement, and warmly sympathized with the evolutionary views of Lamarck and Geoffroy St-Hilaire; it is not therefore to be wondered at that his writings should furnish evidence in favour of each and every interpretation of them. His other morphological labours must not be forgotten. Independently of Vicq d’ Azyr, he discovered the human premaxillary bone; independently of L. Oken, he proposed the vertebral theory of the skull; and before S. C. Savigny, he discerned that the jaws of insects were the limbs of the head.
In 1813 A. P. de Candolle published his Théorie élémentaire de la botanique, which he developed into the classic Organographie végétale (1827). He established his theory of symmetry, reducing all flowers to “symmetrical” groupings of appendages on an axis and accounting for their various forms by cohesion and adhesion, by arrested or excessive development. The next advance was the investigation by W. P. Schimper and A. Braun of phyllotaxis—the ascending spiral arrangement of foliar and floral organs—thus further demonstrating their essential unity.
The term morphology was first introduced by Goethe in 1817, in a subsequent essay (Zur Naturwissenschaft überhaupt, besonders zur Morphologie). It did not come into use in botany until its popularization by Auguste de St-Hilaire in his Morphologie végétale (1841), and in zoology until later, although De Blainville, who also first employed the term type, had treated the external forms of animals under “morphologic.” Though the Naturphilosophie of Schelling and its countless modifications by his followers, its mystic theories of “polarization” and the like, its apparatus of assumption and abstraction, hypothesis and metaphor, cannot here be discussed, its undoubted services must not be forgotten, since it stimulated innumerable reflective minds to the earnest study of natural science, gave a powerful impulse to the study of comparative anatomy and vindicated the claims of philosophic synthesis over those of analytic empiricism. Among its many adherents, some are of more distinctly theological type; others metaphysical, others mystical or poetic, others, again, more especially scientific; but its most typical and picturesque figure is Lorenz Oken, who epitomizes alike the best and the worst features of the school, and among whose innumerable pseudo-morphological dreams there occasionally occurred suggestions of the greatest fruitfulness—notably, for instance, the independent statement of the vertebral theory of the skull.
By far the most distinguished anatomist of the transcendental school is Geoffroy St-Hilaire, who being comparatively free from the extravagances of Oken, and uniting a depth of morphological insight scarcely inferior to that of Goethe with greater knowledge of facts and far wider influence and reputation in the scientific world, had greater influence on the progress of science than either. He started from the same studies of anatomical detail as Cuvier, but, influenced by Buffon’s view of unity of plan and by the evolutionary doctrines of Lamarck, diverged into new lines, and again reached that idea of serial homology of which we have so frequently noted the independent origin. His greatest work, the Philosophie anatomique (1818–1823), contains his principal doctrines. These are: (1) the theory of unity of organic composition, identical in spirit with that of Goethe; (2) the theory of analogues, according to which the same parts, differing only in form and in degree of development, should occur in all animals; (3) the “principe des connexions,” by which similar parts occur everywhere in similar relative positions; and (4) the “principe du balancement des organes,” upon which he founded the study of teratology, and according to which the high development of one organ is allied to diminution of another. The advance in morphological theory is here obvious; unfortunately, however, in eager pursuit of often deceptive homologies, he wandered into the transcendentalism of the Naturphilosophie, and seems utterly to have failed to appreciate either the type theory of Cuvier or the discoveries of Von Baer. He defended Buffon’s and Bonnet’s earlier view of unity of plan in nature; and the controversy reached its climax in 1830, when he maintained the unity of structure in Cephalopods and Vertebrates against Cuvier before the Academy of Sciences. On the point of fact he was of course utterly defeated; the type theory was thenceforward fully accepted and the Naturphilosophie received its death-blow. Such was the popular view; only a few, like the aged Goethe, whose last literary effort was a masterly critique of the controversy, discerned that the very reverse interpretation was the deeper and essential one, that a veritable “scientific revolution” was in progress, and that the supremacy of homological and synthetic over descriptive and analytic studies was thenceforward assured. The irreconcilable feud between the two leaders really involved a reconciliation for their followers; theories of homological anatomy had thenceforward to be strictly subjected to anatomical and embryological verification, while anatomy and embryology acquired a homological aim. This union of the solid matter and rigorous method of Cuvier with the generalizing spirit and philosophic aims of Geoffroy is well illustrated in the works of Owen.
The further evolution of the idea of homology is sketched below, while the extent and rapidity of the subsequent progress of the knowledge of all the structural aspects of plants and animals alike make a historical survey impossible up to the appearance of the Origin of Species (1859). The needful solution was effected by Darwin. The “Urpflanze” of Goethe, the types of Cuvier, and the like, at once became intelligible as schematic representations of ancestral organisms, which in various and varying environments, have undergone differentiation into the vast multitude of existing forms. All the enigmas of structure become resolved; “representative” and “aberrant,” “progressive” and “degraded,” “synthetic” and “isolated,” “persistent” and “prophetic” types no longer baffle comprehension; conformity to type represented by differentiated or rudimentary organs in one organism is no longer contradicted by their entire disappearance in its near allies, while systematist and morphologist become related simply as specialist and generalizer, all through this escape from the Linnaean dogma of the fixity of species. The phenomena of individual development receive interpretation in terms of ancestral history; and embryology thus becomes divided into ontogeny and phylogeny—the latter, too, coming into intimate relation with palaeontology—while classification seeks henceforth the reconstruction of the genealogical tree. All these results were clearly developed in Haeckel’s Generelle Morphologie (1866), while the valuable contemporaneous Principles of Biology of Herbert Spencer also gave special attention to the relation of morphology to physiology.
Individuality.—Probably no subject in the whole range of biology has been more extensively discussed than that of the nature of organic individuality. The history of the controversy is of interest, since besides leading up to solid results it serves, perhaps better than any other case, to illustrate the slow emergence of the natural sciences from the influence of scholastic thought. Starting from the obvious unity and indivisibleness of Man and other higher animals, and adopting some definition such as that of C. F. B. Mirbel, “Tout être organisé, complet dans ses parties, distinct et séparé des autres êtres, est un individu,” it was attempted times without number to discover the same conception elsewhere in nature, or rather to impose it upon all other beings, plants and animals alike. The results of different inquirers were of course utterly discrepant. It seemed easy and natural to identify a tree or herb corresponding to the individual animal, yet difficulties at once arose. Many apparently distinct plants may arise from a common root, or a single plant may be decomposed into branches, twigs, shoots, buds or even leaves, all often capable of separate existence. These, again, are decomposable into tissues and cells, the cells into nucleus, &c., and ultimately into protoplasmic molecules, these finally into atoms—the inquiry thus passing outside organic nature altogether and meeting the old dispute as to the ultimate divisibility of matter. In short, as Haeckel remarks, scarcely any part of the plant can be named which has not been taken by some one for the individual. It is necessary, therefore, briefly to notice some of the principal works on the subject, and these may conveniently be taken in descending order.
While H. Cassini practically agreed with Mirbel in attempting to regard separate plants as individuals, the widest interpretation of the individual is that of G. Gallesio (1816), who proposed to regard as an individual the entire product of a single seed, alike whether this developed into a uni-axial plant extended continuously like a banyan, or multiplied asexually by natural or artificial means like the weeping-willow or the Canadian pondweed, of each of which, on this view, there is only a single individual in Britain, happily discontinuous.
At once the oldest and most frequently maintained view is that which regards the bud or shoot consisting of a single axis with appendages as the plant-individual, of which the tree represents a colony, like a branched hydroid polyp. This conception, often attributed to Aristotle, but apparently without foundation, appears distinctly in the writings of Hippocrates and Theophrastus—the latter saying, “The bud grows on the tree like a plant in the ground.” The aphorism of Linnaeus, “Gemmae totidem herbae,” is well known; and in this view C. F. Wolff and Humboldt concurred, while Erasmus Darwin supported it by an appeal to the facts of anatomy and development. The most influential advocate of the bud theory during the first half of the 19th century was, however, Du Petit-Thouars, who, although starting much as usual with a “principe unique d’existence,” supported his theory on extensive though largely incorrect observations on stem structure and growth. For him the tree is a colony of phytons, each being a bud with its axillant leaf and fraction of the stem and root. Passing over numerous minor authors, we come to the central work of Alex. Braun (1853), in which, as Sachs has clearly pointed out, the illegitimate combination of Naturphilosophie with inductive morphology reaches its extreme. He reviews, however, all preceding theories, admits the difficulty of fixing upon any as final, since the plant, physiologically considered, is rather a dividuum than an individuum, and proposes as a compromise, or indeed as a partial cutting of the knot, the adoption of the shoot, as the morphological individual, comparable to an animal, especially because, unlike the cell, leaf, &c., it includes all the representative characters of the species. Darwin and Spencer on the whole also accept the bud or shoot as at any rate the most definite individual.
The theory of metamorphosis naturally led Goethe, Oken and others to regard the leaf as the individual, while Johannes Müller, J. J. S. Steenstrup and others adopted the same view on various physiological grounds. C. Gaudichaud elaborated a theory intermediate between this view and that of Du Petit-Thouars, according to which the plant was built up of individuals, each consisting of a leaf with its subjacent internode of stem, which was regarded as the leaf-base, and this was supported by Edward Forbes and others, while the nominally converse view—that of the leaf as a mere outward expansion of the stem-segment—was proposed by C. F. Hochstetter.
Though sundry attempts at identifying various tissues, such as the fibro-vascular bundles, as the constituent individuals may be passed over, those associated with the cell theory are of great importance. T. Schwann decided in favour of the cell and regarded the plant as a cell-community, in which the separate elements were like the bees of a swarm—a view virtually concurred in in all essential respects by M. Schleiden, R. Virchow and other founders of the cell theory. Yet, although the structure and functions of the plant are ultimately and specially cellular, it is impossible to ignore the fact that, save in the very lowest organisms, these are subordinated and differentiated into larger aggregates, and form virtually but the bricks of a building, and hence the later theories outlined above. Of attempts to find the individual in the nucleus or the protoplasm granules it is unnecessary to speak further.
So far the theories of absolute individuality. The conception of relative individuality was first clearly expressed by Alphonse de Candolle and Schleiden, both of whom take the cell, the shoot and the multi-axial plant as forming three successive and subordinated categories. K. W. von Nägeli too recognized not only the necessity of establishing such a series (cell, organ, bud, leafy axis, multi-axial plant) but the distinction between morphological and physiological individualities afterwards enunciated by Haeckel.
Passing over the difficulties which arise even among the Protozoa we find that a similar controversy (fully chronicled in Haeckel’s Kalkschwämme) has raged over the individuality of sponges. While the older observers were content to regard each sponge-mass as an individual, a view in which J. N. Lieberkuhn and other monographers substantially concurred, the application of the microscope led to the view suggested by James Clark, and stoutly supported by Saville Kent, that the sponge is a city of amoeboid or infusorian individuals. H. J. Carter looked upon the separate ampullaceous sacs as the true individuals, while others, defining the individual by the possession of a single exhalent aperture, distinguish sponges into solitary and social.
For the higher animals the problem, though perhaps really even more difficult, is less prominent. As Haeckel points out, the earlier discussions and even the comparatively late essay of Johannes Müller take an almost purely psychological or at least a physiological point of view; and the morphological aspect of the inquiry only came forward when the study of much lower forms, such as Cestoid worms (see Platyelmia) or Siphonophores (see Hydrozoa), had raised the difficulties with which botanists had so long been familiar. With the rapid progress of embryology, too, arose new problems; and in 1842 Streenstrup introduced the conception of an “alternation of generations” as a mode of origin of distinct individuals by two methods, for him fundamentally similar, the sexual from impregnated females and the asexual from unimpregnated “nurses”—a view adopted by Edward Forbes and many other naturalists, but keenly criticized by W. B. Carpenter and T. H. Huxley. In R. Leuckart’s remarkable essay on polymorphism (1853) the Siphonophora were analysed into colonies, and their varied organs shown to be morphologically equivalent, while the alternate generations of Steenstrup were reduced to a case of polymorphism in development. Leuckart further partly distinguished individuals of different orders, as well as between morphological and physiological individuals.
In 1852 Huxley, starting from such an undoubted homology as that of the egg-producing process of Hydra with a free-swimming Medusoid, pointed out that the title of individual, if applied to the latter, must logically be due to the former also, and avoided this confusion between organ and individual by defining the individual animal, as Gallesio had done the plant, as the entire product of an impregnated ovum—the swarm of Aphides or free Medusae which in this way might belong to a single individual being termed Zooids.
In Carus’s System of Animal Morphology (1853) another theory was propounded, but the problem then seems to have fallen into abeyance until 1865, when it formed the subject of a prolonged and fruitful discussion in the Principles of Biology. Adopting the cell (defined as an aggregate of the lowest order, itself formed of physiological units) as the morphological unit, H. Spencer points out that these may either exist independently, or gradually exhibit unions into aggregates of the second order, like the lower Algae, of which the individuality may be more or less pronounced. The union of such secondary aggregates or compound units into individuals of a yet higher order is then traced through such intermediate forms as are represented by the higher seaweeds or the liverworts, from the thallus of which the axes and appendages of Monocotyledons and Dicotyledons are ingeniously derived. The shoot of a flowering plant is thus an aggregate of the third order; it branches into an aggregate of the fourth or higher order, and finally as a tree “acquires a degree of composition too complex to be any longer defined.” Proceeding to animals, the same method is applied. The Protozoa are aggregates of the first order. These, like plants, exhibit transitions, of which Radiolarians, Foraminifera and sponges are taken as examples, to such definite compound wholes as Hydra; and such secondary aggregates multiply by gemmation into permanent aggregates of the third order, which may exhibit all degrees of integration up to that of the Siphonophora, where the individualities of the Polyps are almost lost in that of the aggregate form. The whole series of articulated animals are next interpreted as more or less integrated aggregates of the third order, of which the lower Annelids are the less developed forms, the Arthropods the more highly integrated and individualized. Molluscs and vertebrates are regarded as aggregates of the second order.
In 1866 appeared a morphological classic, the Generelle Morphologie of Haeckel. Here pure morphology is distinguished into two sub-sciences—the first purely structural, tectology, which regards the organism as composed of organic individuals of different orders; the second essentially stereometric, promorphology. To tectology, defined as the science of organic individuality, a large section of the work is devoted. Dismissing the theory of absolute individuality as a metaphysical figment, and starting from the view of Schleiden, De Candolle and Nägeli of several successive categories of relative individuals, he distinguishes more clearly than heretofore the physiological individual (or bion), characterized by definiteness and independence of function, from the morphological individual (or morphon), characterized similarly by definiteness of form; of the latter he establishes six categories, as follows:—
1. Plastides (cytodes and cells), or elementary organisms.
In his subsequent monograph on calcareous sponges, and in a final paper, he somewhat modifies these categories by substituting one category of extreme comprehensiveness, that of the idorgan, in place of the three separate orders of organs, antimeres and metameres. The idorgan (of course clearly distinguished from the physiological organ or biorgan) is finally defined as a morphological unit consisting of two or more plastids, which does not possess the positive character of the person or stock. These are distinguished into homoplasts or homo-organs and alloplasts or alloe-organs, the former including, as subdivisions, plastid-aggregates and plastid-fusions, the latter idomeres, antimeres and metameres. The former definition of the term antimere, as denoting at once each separate ray of a radiate, or the right and left halves of a bilaterally symmetrical animal, is corrected by terming each ray a paramere, and its symmetrical halves the antimeres. Thus an ordinary Medusoid has four parameres and eight antimeres, a starfish five and ten. The conception of the persona is largely modified, not only by withdrawing the comparison of the animal with the vegetable shoot and by omitting the antimere and metamere as necessary constituents, but by taking the central embryonic form of all the Metazoa—the gastrula (fig. 1) and its assumed ancestral representative, the gastraea—as the simplest and oldest form of persona. The different morphological stages to which it may attain are classified into three series: (1) Monaxonial inarticulate personae, i.e. uniaxial and unsegmented without antimeres or metameres, as in sponges or lowest Hydroids; (2) Stauraxonial inarticulate personae with antimeres, but without metameres, e.g. coral, medusa, turbellarian, trematode, bryozoon; (3) Stauraxonial articulate personae with antimeres and metameres, e.g. annelids, arthropods, vertebrates. The colonies of protozoa are mere idorgans.
| (After Haeckel.) |
| Fig. 1.—Gastrula in optical section, showing primitive opening and digestive cavity (blastopore and arch-enteron), as also outer and inner layers, ectoderm and endoderm. |
True corms, composed of united personae, occur only among sponges, hydroids, siphonophores, corals, bryozoa, tunicates and echinoderms, of which the apparent parameres are regarded as highly centralized personae of a radially budded worm colony; and these can be classified according to the morphological rank of their constituent personae. They usually arise by gemmation from a single persona, yet in sponges and corals occasionally by fusion of several originally distinct persons or corms. The theory of successive subordinate orders of individuality being thus not only derived from historical criticism of previous theories but brought into conformity with the actual facts of development and descent—various groups of organisms being referred to their several categories—the remaining problem of tectology, that of the relation of the morphological to the physiological individuality, is finally discussed. Of the latter, three categories are proposed: (1) the “actual bion or complete physiological individual,” this being the completely developed organic form which has reached the highest grade of morphological individuality proper to it as a representative of, e.g. its species; (2) the “virtual bion or potential physiological individual,” including any incompletely developed form of the former from the ovum upwards; and (3) the “partial bion or apparent physiological individual,” such fragments of the actual or virtual bion as may possess temporary independence without reproducing the species—this latter category having, however, inferior importance.
Haeckel’s theory, indeed in its earlier form, has been adopted by C. Gegenbaur and other morphologists, also in its later form by G. Jager, who, however, rejects the category of idorgan on the ground of the general morphological principle that every natural body which carries on any chemical changes with its environment becomes differentiated into more or less concentric layers; but the subject, especially as far as animals are concerned, was again discussed in a large work by E. Perrier. Starting from the cell or plastid, he terms a permanent colony a méride, and these may remain isolated like Sagitta or Rotifer, or may multiply by gemmation to form higher aggregates which he terms zoides. Such zoides may be irregular, radiate or linear aggregates, of which the two former classes especially are termed demes. The organ—Haeckel’s idorgan—is excluded, since tissues and organs result from division of labour in the anatomical elements of the mérides, and so have only a secondary individuality, “carefully to be distinguished from the individuality of those parts whose direct grouping has formed the organism, and which live still, or have lived, isolated from one another.” Perrier further points out that the undifferentiated colonies are sessile, as sponges and corals, while a free state of existence is associated with the concentration and integration of the colony into an individual of a higher order.
So far the various theories of the subject; detailed criticism is impossible, but some synthesis and reconciliation must be attempted. Starting from the cell as the morphological unit, we find these forming homogeneous aggregates in some Protozoa and in the early development of the ovum. But integration into a whole, not merely aggregation into a mass, is essential to the idea of individuality; the earliest secondary unit, therefore, is the gastrula or méride. This stage is permanently represented by an unbranched hydroid or sponge or by a planarian. These secondary units may, however, form aggregates either irregular as in most sponges, indefinitely branched as in the hydroids and actinozoa, or linear as in such planarians as Catenula. Such aggregations, colonies or demes, not being aggregated, do not fully reach individuality of the third order. This is attained, however, for the branched series by such forms as Siphonophores among Hydrozoa, or Renilla or Pennatula among Actinozoa; for linear aggregates again by the higher worms, and still more fully by arthropods and vertebrates. Aggregates of a yet higher order may occur, though rarely. A longitudinally dividing Nais or laterally branched Syllis are obviously aggregates of these tertiary units, which, on Haeckel’s view, become integrated in the Echinoderm, which would thus reach a complete individuality of the fourth order. A chain of Salps or a colony of Pyrosoma exhibits an approximation to the same rank, which is more nearly obtained by a radiate group of Botryllus around their central cloaca, while the entire colony of such an ascidian would represent the individual of the fifth order in its incipient and unintegrated state—these and the preceding intermediate forms being, of course, readily intelligible, and indeed, as Spencer has shown, inevitable on the theory of evolution.
The exclusion of tissues and organs from rank in this series is thus seen to necessarily follow. Ectoderm and endoderm cannot exist alone; they and the organs into which they differentiate arise merely, as Jäger expresses it, from that concentric lamination, or, with Perrier, from that polymorphism of the members of the colony, which is associated with organic and social existence. The idea of the antimere is omitted, as being essentially a promorphological conception (for a medusoid or a starfish, though of widely distinct order of individuality, is equally so divisible); that of the metamere is convenient to denote the secondary units of a linear tertiary individual; the term persona, however, seems unlikely to survive, not only on account of its inseparable psychological connotations, but because it has been somewhat vaguely applied alike to aggregates of the second and third order; and the term colony, corm or deme may indifferently be applied to those aggregates of primary, secondary, tertiary or quaternary order which are not, however, integrated into a whole, and do not reach the full individuality of the next higher order. The term zooid is also objectionable as involving the idea of individualized organs, a view natural while the medusoid gonophores of a hydrozoon were looked at as evolved of its homologue in Hydra, whereas the latter may be a degenerate form of the former. Passing to the vegetable world, here, as before, the cell is the unit of the first order, while aggregates representing almost every stage in the insensible evolution of a secondary unit are far more abundant than among animals. Complete unity of the second order can hardly be allowed to the thallus, which Spencer proposes to compound and integrate into tertiary aggregates—the higher plants; as in animals, the embryological method is preferable, both as avoiding gratuitous hypothesis and as leading to direct results. Such a unit is clearly presented by the embryo of higher plants in which the cell-aggregate is at once differentiated into parts and integrated into a whole. Such an embryo possesses axis and appendages as when fully developed (fig. 2). The latter, however, being as organs mere lateral expansions of the concentric layers into which the plant embryo, like the animal, is differentiated, and so neither stages of evolution nor capable of separate existence, are not entitled to individual rank. The embryo, the bud, shoot or uniaxial plant, all thus belong to the second order of individuality, like the hydroid they resemble. Like the lower coelenterates, too, aggregates of such axes are formed by branching out from their low degree of integration. Such colonies can hardly be termed individuals of the third, much less of higher, order, at least without somewhat abandoning that unity of treatment of plants and animals without which philosophical biology disappears. Individuality of the second order is most fully reached by the flower—the most highly differentiated and integrated form of axes and appendages. Such a simple inflorescence as a raceme or umbel approximates to unity of the third order, to which a composite flower-head must be admitted to have attained while a compound inflorescence is on the way to a yet higher stage.
| (After Sachs.) |
| Fig. 2.—Embryo of Dicotyledon, showing incipient axis and appendages, as also the three concentric embryonic layers. |
If, as seems probable, a nomenclature be indispensable for clear expression, it may be simply arranged in conformity with this view. Starting from the unit of the first order, the plastid or monad, and terming any undifferentiated aggregate a deme, we have a monad-deme integrating into a secondary unit or dyad, this rising through dyad-demes, into a triad, this forming triad-demes, and these when differentiated becoming tetrads, the botryllus-colony with which the evolution of compound individuality terminates being a tetrad-deme. The separate living form, whether monad, dyad, triad, or tetrad, requires also some distinguishing name, for which persona will probably ultimately be found most appropriate, since such usage is most in harmony with its inevitable physiological and psychological connotations, while the genealogical individual of Gallesio and Huxley, common also to all the categories, may be designated with Haeckel the ovum-product or ovum-cycle, the complete series of forms needed to represent the species being the species-cycle (though this coincides with the former save in cases where the sexes are separate, or polymorphism occurs). For such a peculiar case as Diplozoon paradoxum, where two separate forms of the same species coalesce, and still more for such heterogeneous individuality as that of a lichen, where a composite unit arises from the union of two altogether distinct forms—fungus and alga—yet additional categories and terms are required.
Promorphology.—Just as the physiologist constantly seeks to interpret the phenomena of function in terms of mechanical, physical, and chemical laws, so the morphologist is tempted to inquire whether organic as well as mineral forms are not alike reducible to simple mathematical law. And just as the crystallographer constructs an ideally perfect mathematical form from an imperfect or fragmentary crystal, so the morphologist has frequently attempted to reduce the complex-curved surfaces of organic beings to definite mathematical expression. Canon Moseley (Phil. Trans., 1838) succeeded in showing, by a combination of measurement and mathematical analysis, that the curved surface of any turbinated or discoid shell might be considered as generated by the revolution, about the axis of the shell, of a curve, which continually varied its dimensions according to the law of the logarithmic spiral. For Goodsir this logarithmic spiral, now carved on his tomb, seemed a fundamental expression of organic curvature and the dawn of a new epoch in natural science—that of the mathematical investigation of organic form—and his own elaborate measurements of the body, its organs, and even its component cells seemed to yield, now the triangle, and again the tetrahedron, as the fundamental form. But such supposed results, savouring more of the Naturphilosophie than of sober mathematics, could only serve to discourage further inquiry and interest in that direction. Thus we find that even the best treatises on botany and zoology abandon the subject, satisfied with merely contrasting the simple geometrical ground-forms of crystals with the highly curved and hopelessly complicated lines and surfaces of the organism.
But there are other considerations which lead up to a mathematical conception of organic form, those namely of symmetry and regularity. These, however, are usually but little developed, botanists since Schleiden contenting themselves with throwing organisms into three groups—first, absolute or regular; second, regular and radiate; third, symmetrical bilaterally or zygomorphic—the last being capable of division into two halves only in a single plane, the second in two or more planes, the first in none at all. H. C. C. Burmeister, and more fully H. G. Bronn, introduced the fundamental improvement of defining the mathematical forms they sought not by the surfaces but by axes and their poles; and Haeckel has developed the subject with an elaborateness of detail and nomenclature which seems unfortunately to have impeded its study and acceptance, but of which the main results may, with slight variations chiefly due to Jäger (Lehrb. d. Zool. i. 283), be briefly outlined.
A. ANAXONIA: Forms destitute of axes, and consequently wholly irregular in form, e.g. Amoebae and many sponges.
B. AXONIA: Forms with definite axes.
I. Homaxonia, all axes equal.
(a) Spheres, where an indefinite number of equal axes can be drawn through the middle point, e.g. Sphaerozoum.
(b) Polyhedra, with a definite number of like axes.
Of these a considerable number occur in nature, for example, many radiolarians (fig. 3), pollen-grains, &c., and they are again classifiable by the number and regularity of their faces.
| Fig. 3.—Radiolarian (Ethmosphaera), an irregular endosphaeric polyhedron with equiangular faces. Type of Homaxonia. | Fig. 4.—Pollen of Passion Flower, as example of Stauraxonia homopola. Ground-form a regular double pyramid of six sides. |
II. Protaxonia, where all the parts are arranged round a main axis, and of these we distinguish—
1. Monaxonia, with not more than one definite axis. Here are distinguished (a) those with similar poles, spheroid (Coccodiscus) and cylinder (Pyrosoma) and (b) those with dissimilar poles, cone (Conulina).
2. Stauraxonia, where, besides the main axes, a definite number of secondary axes are placed at right angles, and the stereometric ground-form becomes a pyramid. Here, again, may be distinguished (a) those with poles similar, Stauraxonia homopola, where the stereometric form is the double pyramid (fig. 4), and (b) those with poles dissimilar, Stauraxonia heteropola, where the stereometric form is the single pyramid, and where we distinguish a basal, usually oral, pole from an apical, aboral or anal pole. The bases of these may be either regular or irregular polygons, and thus a new classification into Homostaura and Heterastaura naturally arises.
The simpler group, the Homostaura, may have either an even or an odd number of sides, and thus among the Homostaura we have even-sided and odd-sided, single and double pyramids. In those Homostaura with an even number of sides, such as medusae, the radial and inter-radial axes have similar poles; but in the series with an odd number of sides, like most echinoderms, each of the transverse axes is half radial and half semi-radial (fig. 5). Of the group of regular double pyramids the twelve-sided pollen-grain of Passiflora (fig. 4) may be taken as an example, having the ground-form of the hexagonal system, the hexagonal dodecahedron. Of the equal even-sided single pyramids (Heteropola homostaura), Alcyonium, Geryonia, Aurelia may be taken as examples of the eight-sided, six-sided, and four-sided, pyramids While those with an odd number of sides may be illustrated by Ophiura or Primula with five sides, and the flower of lily or rush with three sides.
In the highest and most complicated group, the Heterostaura, the basal polygon is no longer regular but amphithect ἀμφίθηκτςο＝double-edged). Such a polygon has an even number of sides, and can be divided into symmetrical halves by each of two planes intersecting at right angles in the middle point, and thus dividing the whole figure into four congruent polygons. The longer of these axes may be termed lateral, the shorter the equatorial or dorsoventral; and these two axes, along with the main axes, always define the three dimensions of space. Ctenophores (fig. 6) furnish examples of eight-sided amphithect pyramids, some Madrepore Corals of six-sided, Crucifers, some Medusae, and Cestodes of four-sided amphithect pyramids.
| Fig. 5.—Starfish, an example of Heteropola homostaura. Ground-form a regular single pyramid of five sides. | Fig. 6.—Ctenophore (Eucharis). Ground-form an eight-sided double amphithect pyramid. | Fig. 7.—Spatangus. Ground form a five-sided half amphithect pyramid. |
In these forms the poles of the dorsoventral and lateral axes are similar, and, as in the preceding Monaxonia and Stauraxonia, the centre of the body is defined by a line; and they are therefore termed Centraxonia, while the Protaxonia which are defined by their central point are called Centrostigma. There are, however, other forms, and these the most complicated, in which the poles of at least the dorso-ventral axis are unlike, and in which the body is thus defined not with reference to a line but to a median plane, and these have accordingly received the name of Centropipeda. Their ground-form is a polygon with an even number of sides, which can only be divided into two symmetrical halves by the one median plane. It can be obtained by halving an amphithect pyramid of double the number of sides, and is consequently termed a half amphithect pyramid (fig. 7). The whole amphithect pyramid may be most conveniently obtained by the reduplication of the ground-form as if in a mirror. Of half amphithect pyramids there are again two forms, termed by Haeckel Amphipleura and Zygopleura, the former including the “bilaterally symmetrical” or irregularly radiate forms of previous authors, such as Spatangus Viola, Orchis, while the Zygopleura include forms bilaterally symmetrical in the strictest sense, in which not more than two radial planes, and these at right angles to each other, are present. The stereometric ground-form is a half rhombic pyramid. Haeckel again divides these, according to the number of antimeres, into Tetrapleura and Dipleura.
Promorphology has thus shown that the reigning dogma of the fundamental difference of organic and mineral forms is false, and that a crystallography of organic forms is possible—the form of the cell or the cell-aggregate differing from the crystal merely by its more or less viscous state of aggregation, its inherited peculiarities, and its greater adaptability to the environment. The classification into bilateral and radiate forms which usually does duty for more precise promorphological conceptions must be abandoned as hopelessly confusing essentially different forms, or at least must be rigidly restricted—the term radial to regular and double pyramids, the term bilateral to the Centropipeda if not indeed to dipleural forms. Similarly the topographical and relative terms, anterior and posterior, upper and under, horizontal and vertical, must be superseded by the terms above applied to the axes and their poles, oral and aboral, dorsal and ventral, right and left.
Nature of Morphological Changes.—The main forms of organic structure being analysed and classified and their stage of individuality being ascertained, the question next arises, by what morphological changes have they arisen, and into what categories can these modes of differentiation be grouped? They at first sight seem innumerable, yet in reality are few. Goethe somewhat vaguely generalized them for the flower as ascending and descending metamorphosis, expansion and contraction of organs, &c.; but the first attempt at careful enumeration seems to be that of Auguste de St Hilaire, who recognized defects of development, adherences, excesses of production or “dédoublements,” metamorphosis and displacement of organs. Subsequent authors have variously treated the subject; thus Asa Gray enumerates as modifications of the flower—coalescence, adnation, irregularity, abortion, non-alternation or anteposition, multiplication, enation, unusual development of the axis, and other morphological modifications connected with fertilization. These are obviously too numerous, as may be best shown by a single comparison with the view of an animal morphologist. Thus Huxley, in discussing the arrangement of the Vertebrata, recognizes only three processes of modification, not only in the ancestral evolution of the Equidae, but in the individual development of animals generally; these are “(1) excess of development of some parts in relation to others, (2) partial or complete suppression of certain parts, (3) coalescence of parts originally distinct”. The particular form of excess of development which results in the repetition of parts, and the morphological changes due to partial or complete fusion of such repeated parts receive special treatment in the article Metamerism.
Nature of Morphological Correspondence—Categories of Homology.—To indicate all the steps by which the idea of morphological resemblance has been distinguished from that of physiological would be to examine the whole history of morphology; it must suffice to discuss the terminology of the subject which has, as ever, served not only as an index but as an engine of progress. For these two distinct forms of resemblance the terms homology and analogy gradually became specialized, and were finally established and clearly defined by Owen in 1843—“the former as the same organ in different animals under every variety of form and function (e.g. fore-limbs of Draco volans and wings of Bird); the second as a part or organ in one animal which has the same function as another part or organ in a different animal (e.g. parachute of Draco and wings of Bird).” He further distinguishes three kinds of homology: (1) special, being “that above defined, 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,” 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,” e.g. the same human bone and centrum of the last cranial vertebra; (3) serial homology, “representative or repetitive relation in the segments of the same skeleton” (demonstrated when general and special homologies have been determined); thus usually the basi-occipital and basi-sphenoid are “homotypes.” These terms were henceforth accepted by naturalists; but the criterion of analogy and homology became for L. Agassiz and other embryologists developmental as well as comparative, reference to the ideal archetype becoming less and less frequent. Passing over the discussions of L. Agassiz and Bronn, 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 corresponding 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 Lankester, who, decomposing it into two others, proposed to supersede the term homology by homogeny, 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. St G. Mivart next proposed 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 both 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, anteroposteriorly 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 Kalkschwämme 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 corresponding, 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 homomorphic 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 “radial,” “vertical,” “lateral” homology, “homotypy of antimeres,” 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 compared are of the same category of individuality—are homacategoric; 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 multicellular 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 homodermic, homosystemic and homodemic, may then conveniently be termed (indifferently save for considerations of priority) either “specially homologous,” “homogeneous,” “homophylic,” or “homogenetic,” 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 “the greatest discovery in the natural sciences of modern times.”
Orientation and Subdivisions of Morphology.—The position of morphology in the classification of the sciences and the proper mode of subdividing it cannot be discussed within these limits, although the latter is especially the subject of much disagreement. The position above assumed, that of including under morphology the whole statical aspects of the organic world, is that of Haeckel, Spencer, Huxley and most recent animal morphologists; botanists frequently, however, still use the term under its earlier and more limited significance (see Plants: Morphology). (P. Ge.; P. C. M.)