LIFE AND IMMORTALITY.

LIFE AND ITS CONDITIONS.

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LL natural objects, roughly divided, arrange themselves into three groups, constituting the so-called Mineral, Vegetable and Animal kingdoms. Mineral bodies are all devoid of life. They consist of either a single element, or, if combined, occur in nature in the form of simple compounds, composed of more than two or three elements. They are homogeneous in texture, or, when unmixed, formed of similar particles which have no definite relations to one another. In form they are either altogether indefinite, when they are said to be amorphous, or have a definite shape, called crystalline, in which case they are ordinarily bounded by plane surfaces and straight lines. When mineral bodies increase in size, as crystals may do, the increase is produced simply by accretion. They exhibit purely physical and chemical phenomena, and show no tendency to periodic changes of any kind. Fossils or petrifactions, which owe their existence and characters to beings which lived in former periods of the earth's history, cannot, though made up of mineral matter, be properly said to belong to the mineral kingdom.

But objects belonging to the vegetable and animal kingdoms differ markedly from inert, lifeless, mineral matter. Carbon, hydrogen, oxygen and nitrogen are the most important of the few chemical elements which enter into

their composition, and these elements are combined into complex organic compounds, which always contain a large percentage of water, are very unstable, and prone to spontaneous decomposition. They are composed of heterogeneous, but related, parts, termed organs, the objects possessing them being called organized bodies. Some of the lowest forms of animals have bodies whose substance is so uniform that they exhibit no definite organs, but this exception does not affect the general value of this distinction. They are always more or less definite in shape, presenting concave and convex surfaces, and being limited by curved lines. When they increase in size, or grow, as we properly term it, it is not by the addition of particles from the outside, but by the reception of foreign matter into their interior and its consequent assimilation. Certain periodic changes, which follow a definite and discoverable order, are invariably passed through by organized bodies. These changes constitute what is known as life. All the objects, then, which fulfil these conditions are said to be alive, and they all appertain either to the vegetable or the animal kingdom. The study of living objects, no matter to which kingdom they belong, is therefore conveniently called by the general name of Biology, which means a discourse on life. And as all living objects may be referred to one or other of these kingdoms, so Biology may be divided into Botany, which treats of plants, and Zoology, which treats of animals.

Now that we have divided all organized bodies into plants and animals, it becomes necessary to inquire into the differences which subsist between them, and which will enable us to separate the kindred sciences of Botany and Zoology. Nothing was thought so easy by older observers than the determination of the animal or vegetable nature of any given organism, but, in point of fact, no hard-and-fast line can be drawn, in the existing state of our knowledge, between the animal and vegetable kingdoms, and it is sometimes difficult,

or even impossible, to decide with positiveness whether we are dealing with a plant or an animal. In the higher orders of the two kingdoms there is no difficulty in reaching a decision, the higher animals being readily separated from the higher plants by the possession of a nervous system, of a locomotive power which can be voluntarily exercised, and of an internal cavity adapted for the reception and digestion of solid food. No so-called nervous system or organs of sense are possessed by the higher plants, although some of them doubtlessly manifest conscious and intelligent action, nor are they capable of voluntary changes of place, nor provided with any definite internal cavity, their food being generally fluid or gaseous.

Descending the scale to the very bottom, we reach a class of animals, the Protozoa, which cannot be separated in many cases from the Protophyta by these distinctions, since many of the former have no digestive cavity, nor the slightest trace of a nervous system, while many of the latter possess the power of active locomotion. As to external configuration, no certain rules can be laid down for separating animals and plants, many of the lower plants, either in their earlier stages, or in their maturity, being exactly similar in form to some of the lower animals. This is the case with some of the Algæ, which resemble very closely in form certain Infusorian animalcules. Again, many undoubted animals, which are rooted to solid objects in their adult state, are so plant-like in appearance as to be popularly regarded as vegetables. The Sea-firs, and the more highly organized Flustras or Seamats, which are usually considered as sea-weeds by sea-side visitors, are a few of many examples that might be taken from the so-called Hydroid Zoophytes. No decided distinction between animals and plants can be drawn as to their minute internal structure, both alike consisting of molecules, of cells, or of fibres. Some decided, though not universal, differences exist in chemical composition. Plants exhibit a decided predominance of ternary compounds, or compounds

which, like sugar, starch and cellulose, are made up of the three elements, carbon, hydrogen and oxygen, but are, comparatively speaking, poorly supplied with quaternary compounds, or those which contain an additional element of nitrogen. Animals, on the contrary, are rich in quaternary nitrogenized compounds, such as albumen or fibrin. Still, in both kingdoms we find nitrogenized and non-nitrogenized compounds, and it is only in the proportion which these sustain to each other in the organism that animals differ in any way from plants.

Before the invention of the microscope, no independent voluntary movements, if we except the opening and closure of flowers, and their turning towards the sun, the drooping of the leaves of sensitive plants under irritation, and some other kindred phenomena, were known in plants. Now, however, we know of many plants which are endowed, either when young or throughout life, with the power of effecting voluntary movements apparently as spontaneous and independent as those performed by the lower animals, the movenents being brought about by means of little vibrating cilia, or hairs, with which a part or the whole of the surface is furnished. When it is added that many animals are permanently rooted, in their fully-grown condition, to solid objects, it will at once be apparent that no absolute distinction can be made between animals and plants merely because of the presence or absence of independent locomotive power.

There is, however, a test, the most reliable of all that have been discovered, by which an animal may be distinguished from a plant, and that is the nature of the food and the products which are elaborated therefrom in the body. Plants live upon such inorganic substances as water, carbonic acid and ammonia, and they have the power of manufacturing out of these true organic materials, and are therefore the great producers of nature. All plants which contain green coloring matter, technically called chlorophyll, break up carbonic acid in the process of digestion into its two

constituents of carbon and oxygen, retaining the former and setting the latter free. And as the atmosphere always contains carbonic acid in small quantities, the result is that plants remove carbonic acid therefrom and give out oxygen. Animals, on the other hand, have no power of living on water, carbonic acid and ammonia, nor of converting these into the complex organic substances of their bodies. That their existence may be maintained animals require to be supplied with ready-made organic compounds, and for these they are all dependent upon plants, either directly or indirectly. In requiring as food complex organic bodies, which they ultimately reduce to very simply inorganic ones, animals are thus found to differ from plants. Whilst plants are the great manufacturers in nature, animals are the great consumers. Another distinction, arising from the nature of their food, is that animals absorb oxygen and throw out carbonic acid, their reaction upon the atmosphere being exactly the reverse of that of plants. There are organisms, it must be understood, which are genuine plants so far as their nutritive processes are concerned, but which, nevertheless, are in the possession of characters which could locate them among the animals. Volvox, so abundant in our streams during the proper seasons, affords a splendid illustration of the truth of this statement. Plants, which are devoid of chlorophyll, as is the case with the Fungi, do not possess the power of decomposing carbonic acid under the influence of sunlight, but are like animals in requiring organic compounds for their food. Two points must therefore be borne in mind in regarding the general distinctions between plants and animals which we have thus briefly outlined, and these are that they cannot often be applied in practice to ambiguous microscopic organisms, and certainly not to plant-forms that are destitute of chlorophyll.

That life should manifest itself certain conditions are essential, but some of which, though generally present, are not absolutely indispensable. One condition, however, seems to