To the two groups of unarticulated and articulated sprouts in the kingdom of the tissue-plants correspond, in many respects, the two sections of the tissue-animals, the unarticulated and the articulated. The two stems of the articulates and vertebrates rise above all the other metazoa by the perfection of their organism and the variety of their functions. In the articulates the metamerism is chiefly external—an articulation of the body wall. In the vertebrates it mainly affects the internal organs, the skeleton, and the muscular system. The vertebration (articulation) of the vertebrates is not outwardly visible like that of the articulates. In both stems the articulation is similar in the lower and upper forms, as we find in the annelids and myriapods, the acrania and cyclostoma. On the other hand, the higher the organization the greater is the unlikeness of the members or articulated parts, as in the arachnida and insects, the amphibia and amniotes. The same antithesis is found in the lower and higher crustacea. This metamerism of the higher metazoa is of a motor character, having been acquired through the manner of movement of the lengthened body; but we find in some groups of the lower, and usually unarticulated, metazoa a propagative metamerism, determined by budding at the end; such is the strobilation of the chain-worms and the scyphostoma polyps. The individual metamera (parts) that are released from the end of the chain in these cases immediately show their individuality. This is also the case with many of the annelids, in which every member that is separated has the power to reproduce the whole chain of metamera.
The third and highest stage of individuality to which the multicellular organism attains is the stock or colony (cormus). It is usually formed by a permanent association of histonals that are produced by cleavage (imperfect segmentation or budding) from one histonal individual. The great majority of the metaphyta form complex plants in this sense. But among the metazoa we find this form of individuality only in the lower (and generally stationary) stages of development. Here also there is a striking parallelism of development between the two chief groups of the histona. At the lower stages of stock-formation there is equality of the social histonals. But in the higher grades they become unequally developed in the division of labor; and the greater the differences between them become, the greater is the centralization of the whole stock (as in the case of the siphonophora). We may therefore distinguish two principal forms of stocks—the homonomous and heteronomous, the one without, and the other with, division of labor among the histonals.
The history of civilization teaches us that its gradual evolution is bound up with three different processes: (1) Association of individuals in a community; (2) division of labor (ergonomy) among the social elements, and a consequent differentiation of structure (polymorphism); (3) centralization or integration of the unified whole, or rigid organization of the community. The same fundamental laws of sociology hold good for association throughout the entire organic world; and also for the gradual evolution of the several organs out of the tissues and cell-communities. The formation of human societies is directly connected with the gregariousness of the nearest related mammals. The herds of apes and ungulates, the packs of wolves, the flocks of birds, often controlled by a single leader, exhibit various stages of social formation; as also the swarms of the higher articulates (insects, crustacea), especially communities of ants and termites, swarms of bees, etc. These organized communities of free individuals are distinguished from the stationary colonies of the lower animals chiefly by the circumstance that the social elements are not bodily connected, but held together by the ideal link of common interest.
VIII
FORMS OF LIFE
Morphology—Laws of symmetry—Fundamental forms of animals and plants—Fundamental forms of protists and histona—Four chief classes of fundamental forms: (1) Centrostigma: vesicles (smooth vesicle and tabular vesicle); (2) Centraxonia: typical forms with central axis—Uniaxial (monaxonia, equipolar and unequipolar)—Transverse-axial (stauraxonia, double-pyramidal and pyramidal); (3) Centroplana: fundamental forms with central plane—Bilateral symmetry—Bilateral-radial and bilateral-symmetrical fundamental forms—Asymmetrical fundamental forms; (4) Anaxonia: irregular fundamental forms—- Causes of form-construction—Fundamental forms of monera, protists, and histona—Fundamental form and mode of life—Beauty of natural forms—Æsthetics of organic forms—Art forms in nature.
The infinite variety of forms which we observe in the realm of organic life not only delight our senses with their beauty and diversity, but also excite our curiosity, in suggesting the problem of their origin and connection. While the æsthetic study of the forms of life provides inexhaustible material for the plastic arts, the scientific study of their relations, their structures, their origin and evolution, forms a special branch of biology, the science of forms or morphology. I expounded the principles of this science in my General Morphology thirty-eight years ago. They are so remote from the ordinary curriculum of education, and are so difficult to explain without the aid of numerous illustrations, that I cannot think of going fully into them here. In the present chapter I will only briefly describe those features of living things which relate to the difficult question of their ideal fundamental forms, the laws of their symmetry, and their relation to crystal-formation. I have treated these intricate questions somewhat fully in the last (eleventh) part of Art-forms in Nature. The hundred plates contained in this work may serve as illustrations of morphological relations. In the following pages the respective plates are indicated by the letters A-f, with the number of each.
The unity of the organic structure, which expresses itself everywhere in the fundamental features of living things and in the chemical composition and constructive power of their plasm, is also seen in the laws of symmetry in their typical forms. The infinite variety of the species may, both in the animal and plant worlds, be reduced to a few principal groups or classes of fundamental forms, and these show no difference in the two kingdoms (cf. plate 6). The lily has the same regular typical form as the hexaradial coral or anemone (A-f, 9, 49), and the bilateral-radial form is the same in the violet and the sea-urchin (clypeaster, A-f, 30). The dorsiventral or bilateral-symmetrical form of most green leaves is repeated in the frame of most of the higher animals (the cœlomaria); the distinction of right and left determines in each the characteristic antithesis of back and belly.