animal, as Gallesio had done the plant, as the entire product of an impregnated ovum-the swarm of Aphides or free Medusæ 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 gradua.lly 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 gem mat ion 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 stereo metric, pro morphology. 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 Nageli 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:- l. Plaslides (cytodes and cells), or elementary organisms. 2. Organs (cell-stocks or cell-fusions), simple or homo plastic organs (tissues), or hetero plastic organs. Organ-systems, organ-apparatuses.

3. Antimeres (opposite or symmetrical or homotypic parts), e.g. rays of radiate animals, “ halves of bilaterally symmetrical animals."

4. Metameres (successive or homodynamous parts), e.g. stem segments of Phanerogams, segments or zoonites of Annelids or vertebrates.

5. Personae, shoots or buds of plants, polyps of Coelenterates, &c., “ individuals " in the narrowest sense among the higher animals.

6. Corms (stocks or colonies), e.g. trees, chains of Salpae, polyp stocks, &c.

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, anti meres and meta meres. 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 alloc-organs, the former including, as subdivisions, plastid-aggregates and plastidfusions, the latter idomeres, anti meres and meta meres. 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 anti meres. Thus an ordinary Medusoid has four para meres and eight anti meres, 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. I) 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: (I) Monaxonial inarticulate personae, i.e. uniaxial and unsegmented without anti meres or meta meres, as in spongles or lowest Hydroids; (2) Stauraxonial inarticulate personae wit anti meres, but without meta meres, ag. coral, medusa, turbellarian, trematode, bryozoon; (3) Stauraxonia articulate personae with anti meres and meta meres, e.g. annelids, arthropods, vertebrates. The colonies of protozoa are mere idorgans. True corms, composed of united personae, occur only among sponges, hydroids, siphonophores, corals, bryozoa, tunic ates and echinoderms, of which the apparent para meres are* regarded as highly

centralized personae of a radially budded

2°

r

gig.

/ f 1

ltr ” <»

(After Haeckel.)

FIG. I.-Gastrula in

optical section, showing

primitive opening and

digestive cavity (blastopore

and arch-enteron),

as also outer and inner

layers, ectoderm and endoderm.

worm colony; and these can be classified

according to the morphological rank of

their constituent personae. They usually

arise by gem mat ion 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

tectol0§ Y, that of the relation of the

morphological to the physiological individuality, is finally discussed. Of the

latter, three categories are proposed:

(I) 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 mérlde, and these may remain isolated like Sagitta or Rotifer, or may multiply by gem mat ion 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 ziemes. The organ-Haeckels 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 Calenula. Such aggregations, colonies or demes, not being aggregated, do not fully reach individuality of the third order. This is attained, ho'wever, 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 arthropod sand vertebrates. Aggregates of a yet higher order may occur, though rarely. A longitudinally dividing 'Na/is 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 stat¢%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