PART III THE FORMS OF ANIMAL LIFE CHAPTER XI THE ELEMENTS OF STRUCTURE 1. The Resemblances and Contrasts between Plants and Animals— 2. The Relation of the simplest Animals to those which are more complex-3. The Parts of the Animal Body: Organs, Tissues, Cells THE study of form and structure (Morphology), and the study of habit and function (Physiology), are both as essential to science as the realities are in life. It is with the forms of animal life and their structure that we have now to do, but it seems useful at the outset to compare plants and animals. 1. The Resemblances and Contrasts between Plants and Animals. Every one could point out some differences between a tree and a horse, but many might be puzzled to distinguish clearly between a sponge and a mushroom, while all have to confess their inability to draw a firm line between the simplest plants and the simplest animals. For the tree of life is double like the letter V, with divergent branches, the ends of which, represented, let us say, by a daisy and a bird, are far apart, while the bases gradually approach and unite in a common root. Plants and animals are alike, though not equally, alive. Diverse as are the styles of animal and vegetable architecture, the materials are virtually the same, and the individuals in both cases grow from equally simple beginnings. Even movement, the chief characteristic of animals, occurs commonly, though in a less degree, among plants. Young shoots move round in leisurely circles, twining stems and tendrils bend and bow as they climb, leaves rise and sink, flowers open and close with the growing and waning light of day. Tendrils twine round the lightest threads, the leaves of the sensitive plant respond to a gentle touch, the tentacles of the sundew and the hairs of the fly-trap compare well with the sensitive structures of many animals. The stamens of not a few flowers move when jostled by the legs of insects, and the stigma of the musk closes on the pollen. We Plants and animals alike consist of cells or unit masses of living matter. The structure of the cell and the apparent structure of the living stuff is much the same in both. may liken plants and animals to two analogous manufactories, both very complex; we study the raw materials which pass in, many of the stages and by-products of manufacture, and the waste which is laid aside or thrown out, but in neither case can we enter the secret room where the mystery of the process is hidden. In the pond we find the eggs of water-snails and waterinsects attached to the floating leaves of plants; in the ditches in spring we see in many places the abundant spawn of frogs and toads; we are familiar with the heavily yolk-laden eggs of birds. Now, with a little care it is quite easy to convince ourselves that an egg or ovum is to begin with a simple mass of matter, in part, at least, alive, and that by division after division the egg gives rise to a young animal. We are also well aware that in most cases the egg-cell, for cell it is, only begins to divide after it has been penetrated, and in some subtle way stimulated, by a male unit or sperm. The great facts of individual history or development then are, the apparent simplicity in the beginning, the preliminary condition that the egg-cell be united with a male unit, and the mode of growth by repeated division of the ovum and its daughter-cells. In those plants with which we are most familiar, the facts seem different, for we watch bean and oak growing from seeds which, instead of being simple units, are very complex structures. But the seed is not the beginning of a plant, it has already a long history behind it, and when that history is traced back to the seedbox and possible seeds of the parent plant, there it will be seen that the beginning of the future herb or tree is a single cell. This is the equivalent of the animal ovum, and, like it, begins its course of repeated divisions after it has been joined by a kernel or nucleus from the pollen grain. Thus, to sum up, along three different paths we reach the same conclusion, that there is a fundamental unity between plants and animals. In the essential activities of their life, in the stones and mortar of their structure, and lastly, in the way in which each individual begins and grows, there is a real unity. Yet, after all, plants and animals are very different. The two kinds of organisms may be ranked as two great branches of one tree of life, yet the branches diverge widely and bear different foliage. The facts of divergence and diversity are as undeniable as the inseparable unity of the basal trunk and the genuine sameness of life throughout the whole tree. I have stated the chief contrasts between plants and animals in a tabulated summary and works in the animal. The net result of this contrast is that animals are more active than plants. Life slumbers in the plant; it wakes The changes associated with the living matter of an animal are seemingly more intense and rapid; the ratio of disruptive, power-expending changes to constructive power-accumulating changes is greater; most animals live more nearly up to their income than most plants do. They live on richer food; they take the pounds which plants have accumulated in pence, and spend them. Of course plants also expend energy, but for the most part within their own bodies; they neither toil nor spin. They stoop to conquer the elements of the inorganic world, but have comparatively little power of moving or feeling. They are more conservative and miserly than the liberally spendthrift animals, and it is possible that some of the most characteristic possessions of plants, e.g. cellulose, may be chemical expressions of a marked preponderance of constructive and up-building vital processes. It is enough, however, if we have to some extent realised the common'places that plants and animals live the same sort of life, but that the animals are on an average more active and wide-awake than the plants. 2. The Relation of the Simplest Animals to those which are more Complex.-From the pond-water catch in a glass tube one of the small animals, suppose it be a tiny water-flea or a minute "worm"; how does it differ from one of the simplest animals, such as an Infusorian? It consists of many units of living matter instead of only one. The contrast is like that between an egg and the bird which is hatched from within it. The simplest animals are single cells, all the others from sponge to man are many-celled. The Protozoa are units; all others—the Metazoa- are composite aggregates of units, or cities of cells. Compare the life of one of the Protozoa with that of a worm, a frog, or a bird. Both are alive, both may be seen moving, shrinking away from what is hurtful, drawing near to what is useful, engulfing food, and getting rid of refuse. Both are breathing, for carbonic acid will poison them, and dearth of oxygen will kill them; both grow and |