Herbert Spencer

“ 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. ”

“ Converse facts are presented after development has been completed; for with the commencement of reproduction we everywhere see more or less resumption of individual life among the units, or some of them. It is a trait of transitional types between Protozoa and Metazoa to lead an aggregate life as a plasmodium, and then for this to break up into its members, which for a time lead individual lives as generative agents; and sundry low kinds of plants possessing small amounts of structure, have generative elements—zoospores and spermatozoids—which show us a return to unit life. ”

“ Unobtrusive, and yet to be found at every point, are the ducts of the lymphatic system. And in all parts exists the connective tissue—an inert tough substance which, running through interspaces, wraps up and binds together the other tissues. As is implied by this description, these structures stand in contrast with local structures. Here is a bone, there is a muscle, in this place a gland, in that a sense-organ. Each has a limited extent and a particular duty. But through every one of them ramify branches of these universal structures. ”

“ There is the fundamental fact that the plants and animals with which we are familiar—Metaphyta and Metazoa—are formed by the aggregation of units homologous with Protozoa. These units, often conspicuously showing their homology in early embryonic stages, continue some of them to show it throughout the lives of the highest type of Metazoa, which contain billions of units carrying on a protozoon life. Of the protoplasts not thus active the great mass, comparatively little transformed in low organisms, become more and more transformed as the ascent to high organisms goes on ”

“ On going back to the time when the organism begins with a single cell, then becomes a spherical cluster of cells, and then exhibits differences in the modes of aggregation of these cells, the first conspicuous rise of structure (limiting ourselves to animals) is the formation of three layers. Of these the first is, at the outset and always, the superficial layer in direct contact with the environment. The second, being originally a part of the first, is also in primitive types in contact with the environment, but, being presently introverted, forms the rudiment of the food-cavity ”

“ In the order of decreasing generality an account of organs should be preceded by an account of systems of organs. Some of these, as the muscular system and the osseous system, are co-extensive with tissues, but others of them are not. The nervous system, for example, contains more than one kind of tissue and is constituted of many different structures: besides afferent and efferent nerves there are the ganglia immediately controlling the viscera, and there are the spinal and cerebral masses, the last of which is divisible into numerous unlike parts. ”

“ Even in the highest types, however, and even when they are fully developed, unit-life does not wholly disappear: it is clearly shown in ourselves. I do not refer simply to the fact that, as throughout the animal kingdom at large and a considerable part of the vegetal kingdom, the male generative elements are units which have resumed the primitive independent life, but I refer to a much more general fact. ”

“ They are divisible into the universal and the particular—those which are everywhere present and those which occupy special places. The blood which a scratch brings out shows us that the vascular system sends branches into each spot. ”

“ 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. ”

“ This process of elaborating organs so as to meet additional requirements by additional parts, is the process pursued throughout the body at large.
Of plant-structures, concerning which so little has been said, it may here be remarked that their relative simplicity is due to the simplicity of their relations to food. The food of plants is universally distributed, while that of animals is dispersed. The immediate consequences are that in the one case motion and locomotion are superfluous, while in the other case they are necessary: the differences in the degrees of structure being consequences. ”

“ But in the Metazoa and Metaphyta at large, the associated units are, with the exceptions just indicated, completely subordinated. The unit-life is so far lost in the aggregate life that neither locomotion nor the relative motion of parts remains; and neither in shape nor composition is there resemblance to protozoa. Though in many cases the internal protoplasm continues to carry on vital processes subserving the needs of the aggregate, in others vital processes of an independent kind appear to cease. ”

“ This remainder truth is that, different as they may be in the rest of their structures, all organs are alike in certain of their structures. All are furnished with these appliances for nutrition, depuration and excitation: they have all to be sustained, all to be stimulated, all to be kept clean. It has finally to be remarked that the general structures which pervade all the special structures at the same time pervade one another. The universal nervous system has everywhere ramifying through it the universal vascular system which feeds it ”

“ But if the cells forming a portion of tissue are subject to greater pressure than the cells around, their contained protoplasm must be forced through the connecting threads into these surrounding cells. Every change of pressure at every point must cause movements and counter-movements of this kind. Now in the Metazoa at large, or at least in all exhibiting relative motions of parts, and especially in all which are capable of rapid locomotion, such changes of pressure are everywhere and always taking place. ”

“ It suffices that the white corpuscles or leucocytes, retaining the primitive amœboid character, exhibit individual activities: send out prolongations like pseudopodia, take in organic particles as food, and are independently locomotive. Though far less numerous than the red corpuscles, yet, as ten thousand are contained in a cubic millimetre of blood—a mass less than a pin's head—it results that the human body is pervaded throughout all its blood-vessels by billions of these separately living units. ”

“ Contrariwise, the nutriment animals require is distributed through space in portions: in some cases near one another and in other cases wide apart. Hence motion and locomotion are necessitated; and the implication is that animals must have organs which render them possible. In the first place there must be either limbs or such structures as those which in fish, snakes, and worms move the body along. ”

“ The cell-contained protoplasts thus variously changed are fused together into tissues in which their individualities are practically lost; but they nevertheless remain connected throughout by permeable strands of protoplasm. Arising by complication of the outer and inner layers of the embryo and growing more unlike as their units become more obscured, these tissues are formed into systems, which develop into sets of organs. Some of the resulting structures are localized and special but others are everywhere interfused. ”

“ So vast and varied a subject as organic structure, even though the treatment of it is limited to the enunciation of principles, cannot, of course, be dealt with in the space here assigned. Next to nothing has been said about plant-structures, and in setting forth the leading traits of animal-structures the illustrations given have been mostly taken from highly-developed creatures. ”

“ 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. ”

“ The contraction of a muscle, besides compressing its components, compresses neighbouring tissues; and every instant contractions and relaxations of muscles go on throughout the limbs and body during active exertion. ”

“ With the advance of development the unit-life diminishes; but still, in plants, recommences where the disintegrating process which initiates generation shows the coercive power of the organization to have become small. ”

“ Bridges of protoplasm have been seen between epithelium-cells, and it is maintained that cartilage-cells, connective tissue cells, the cells forming muscle-fibres, as well as nerve-cells, have protoplasmic unions. ”

“ These different kinds of evidence jointly imply that the individual lives of the units are subordinate to the general life in proportion as this is high. Where the organism is very inferior in type the unit-life remains permanently conspicuous. In some superior types there is a display of unit-life during embryonic stages in which the co-ordinating action of the aggregate is but incipient. ”

“ There is a femur which helps its fellow to support the weight of the body; and there again is a rib which, along with others, forms a protective box for certain of the viscera. Dissection reveals a set of muscles serving to straighten and bend the fingers, certain other muscles that move the legs, and some inconspicuous muscles which, contracting every two or three seconds, slightly raise the ribs and aid in inflating the lungs. ”

“ In the fourth place there must be organs for separating and excreting waste-products. All these appliances must be more highly developed in proportion as the required activity is greater. Then there must be an apparatus for directing the motions and locomotions—a nervous system; and as fast as these become rapid and complex the nervous system must be largely developed, ending in great nervous centres—seats of intelligence by which the activities at large are regulated. Lastly, underlying all the structural contrasts between plants and animals thus originating, there is the chemical contrast ”

“ So that, strange as it seems, those chemical, physical, and mental characters of animals which so profoundly distinguish them from plants, are all remote results of the circumstance that their food is dispersed instead of being everywhere present. ”

“ An inspiration alters the stress on the tissues throughout a considerable part of the trunk, and a heart-beat propels, down to the smallest arteries, waves which slightly strain the tissues at large. The component cells, thus subject to mechanical disturbances, small and great, perpetual and occasional, are ever having protoplasm forced into them and forced out of them. ”

“ Moreover, each attitude—standing, sitting, lying down, turning over—entails a different set of pressures, both of the parts on one another and on the ground; and those partial arrests of motion which result from sitting down the feet alternately when running, send jolts or waves of varying pressure through the body. ”

“ 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. ”

“ 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 all, lining the channels which permeate the mass, and driving onwards the contained sea-water by the motions of their whip-like appendages, substantially retain their separate lives ”

“ The physiological character of epithelia is given in the fact that they cover the surfaces of the body, their morphological character in that they consist of closely compressed cells united only by a cementing substance. ”

“ Recognizing the locomotive powers of minute Algæ and the motions of such other Algæ as Oscillatoria, as well as those movements of leaves and fructifying organs seen in some Phænogams, we may say, generally, that plants are motionless; but that they can nevertheless carry on their lives because they are bathed by the required nutriment in the air and in the soil. ”