CHAPTER IIA.
STRUCTURE.21
§ 54a. As, in the course of evolution, we rise from the smallest to the largest aggregates by a process of integration, so do we rise by a process of differentiation from the simplest to the most complex aggregates. The initial types of life are at once extremely small and almost structureless. Passing over those which swarm in the air, the water, and the soil, and are now some of them found to be causes of diseases, we may set out with those ordinarily called Protozoa and Protophyta: the lowest of which, however, are either at once plants and animals, or are now one and now the other.
That the first living things were minute portions of simple protoplasm is implied by the general theory of Evolution; but we have no evidence that such portions exist now. Even admitting that there are protoplasts (using this word to include plant and animal types) which are without nuclei, still they are not homogeneous – they are granular. Whether a nucleus is always present is a question still undecided; but in any case the types from which it is absent are extremely exceptional. Thus the most general structural traits of protoplasts are – the possession of an internal part, morphologically central though often not centrally situated, a general mass of protoplasm surrounding it, and an inclosing differentiated portion in contact with the environment. These essential elements are severally subject to various complications.
In some simple types the limiting layer or cortical substance can scarcely be said to exist as a separate element. The exoplasm, distinguished from the endoplasm by absence or paucity of granules, is continually changing places with it by the sending out of pseudopodia which are presently drawn back into the general mass: the inner and outer, being unsettled in position, are not permanently differentiated. Then we have types, exemplified by Lithamœba, constituted of protoplasm covered by a distinct pellicle, which in sundry groups becomes an outer shell of various structure: now jelly-like, now of cellulose, now siliceous or calcareous. While here this envelope has a single opening, there it is perforated all over – a fenestrated shell. In some cases an external layer is formed of agglutinated sand-particles; in others of imbricated plates, as in Coccospheres; and in many others radiating spicules stand out on all sides. Throughout sundry classes the exoplasm develops cilia, by the wavings of which the creatures are propelled through the water – cilia which may be either general or local. And then this cortical layer, instead of being spherical or spheroidal, may become plano-spiral, cyclical, crosier-shaped, and often many-chambered; whence there is a transition to colonies.
Meanwhile the inclosed protoplasm, at first little more than a network or foamwork containing granules and made irregular by objects drawn in as nutriment, becomes variously complicated. In some low types its continuity is broken by motionless, vacant spaces, but in higher types there are contractile vacuoles slowly pulsing, and, as we may suppose, moving the contained liquid hither and thither; while there are types having many passive vacuoles along with a few active ones. In some varieties the protruded parts, or pseudopodia, into which the protoplasm continually shapes itself, are comparatively short and club-shaped; in others they are long and fine filaments which anastomose, so forming a network running here and there into little pools of protoplasm. Then there are kinds in which the protoplasm streams up and down the protruding spicules: sometimes inside of them, sometimes outside. Always, too, there is included in the protoplasm a small body known as a centrosome.
Lastly, we have the innermost element, considered the essential element – the nucleus. According to Prof. Lankester, it is absent from Archerina, and there are types in which it is made visible only by the aid of special reagents. Ordinarily it is marked off from the surrounding protoplasm by a delicate membrane, just as the protoplasm itself is marked off by the exoplasm from the environment. Most commonly there is a single nucleus, but occasionally there are many, and sometimes there is a chief one with minor ones. Moreover, within the nucleus itself there have of late years been discovered remarkable structural elements which undergo complicated changes.
These brief statements indicate only the most general traits of an immense variety of structures – so immense a variety that Prof. Lankester, in distinguishing the classes, sub-classes, orders, and genera in the briefest way, occupies 37 quarto pages of small type. And to give a corresponding account of Protophyta would require probably something like equal space. Thus these living things, so minute that unaided vision fails to disclose them, constitute a world exhibiting varieties of structure which it requires the devotion of a life to become fully acquainted with.
§ 54b. If higher forms of life have arisen from lower forms by evolution, the implication is that there must once have existed, if there do not still exist, transitional forms; and there follows the comment that there do still exist transitional forms. Both in the plant-world and in the animal-world there are types in which we see little more than simple assemblages of Protophyta or of Protozoa– types in which the units, though coherent, are not differentiated but constitute a uniform mass. In treating of structure we are not here concerned with these unstructured types, but may pass on to those aggregates of protoplasts which show us differentiated parts —Metaphyta and Metazoa: economizing space by limiting our attention chiefly to the last.
When, half a century ago, some currency was given to the statement that all kinds of organisms, plant and animal, which our unaided eyes disclose, are severally composed of myriads of living units, some of them partially, if not completely, independent, and that thus a man is a vast nation of minute individuals of which some are relatively passive and others relatively active, the statement met, here with incredulity and there with a shudder. But what was then thought a preposterous assertion has now come to be an accepted truth.
Along with gradual establishment of this truth has gone gradual modification in the form under which it was originally asserted. If some inhabitant of another sphere were to describe one of our towns as composed exclusively of houses, saying nothing of the contained beings who had built them and lived in them, we should say that he had made a profound error in recognizing only the inanimate elements of the town and disregarding the animate elements. Early histologists made an analogous error. Plants and animals were found to consist of minute members, each of which appeared to be simply a wall inclosing a cavity – a cell. But further investigation proved that the content of the cell, presently distinguished as protoplasm, is its essential living part, and that the cell-wall, when present, is produced by it. Thus the unit of composition is a protoplast, usually enclosed, with its contained nucleus and centrosome.
§ 54c. As above implied, the individualities of the units are not wholly lost in the individuality of the aggregate, but continue, some of them, to be displayed in various degrees: the great majority of them losing their individualities more and more as the type of the aggregate becomes higher.
In a slightly organized Metazoon like the sponge, the subordination is but small. Only those members of the aggregate which, flattened and united together, form the outer layer and those which become metamorphosed into spicules, have entirely lost their original activities. Of the rest nearly