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
