Part 18

True flight seems to need a definite mechanism of one kind--namely, a mechanism which shall give rapid and reiterated blows to the air from a point towards the dorsal side, and head end of the body, by structures of considerable superficial extent, and capable of rapid and delicate inclinations of surface. Such structures must be light and therefore delicate, and yet possess very considerable strength to resist the strain of the body's prolonged sustentation, and to effect its occasionally very rapid progress, as in the swift and in dragon-flies. These conditions which we find fulfilled in all existing flying organisms were also fulfilled organisms which have for ages passed away from the surface of this by planet, such as the extinct flying reptiles called _Pterosauria_ or _Pterodactyles_.[82]

In all such rapidly flying creatures the form of the body is necessarily modified so as to throw the centre of gravity where it may be best sustained. It is this which packs what are practically a bird's teeth in its belly, and thickens so greatly the muscles on its breast which are formed in such a way as to serve both the usual purposes of breast-muscles, and also that which is effected in most cases by muscles of the back, which in birds are very greatly diminished in volume and extent.

But there are living creatures which have relations with two media; which, though they are aquatic, yet by the help of the air rise and float, so as to be partly bathed in the atmosphere; while others carry down a portion of that atmosphere below the surface of water, so as to be sub-aqueously aërial. Examples of the last-mentioned condition are afforded by such spiders as have the habit of enclosing a bubble of air within the meshes of their self-woven network, and going down with it, being thus able there to maintain themselves as in a diving-bell. The reverse condition obtains in such plants as _Valisneria_,[83] which secrete air within expanded bladder-like receptacles, and, thus aided, rise to the surface and float. Another example is that of certain polyp animals, such as the Portuguese man of war, which also rise and swim upon the surface of the sea by the aid of floats in the form of bladders, which are also filled with air by means of their own life processes. The same also is the case in many seaweeds.

Thus, these multitudinous forms of living creatures, both animals and plants, are reducible to certain categories in harmony with their modes of life, and the relations existing between them and all surrounding influences. We may see that, without compliance with certain of such laws, their existence would be impossible, and we see that there is a general correspondence between their shape and structure on the one hand, and their environment (that is, the totality of all surrounding agencies and influences) on the other. Are we to consider that such influences are the _causes_ of their form and structure? Obviously the biological facts before us, as yet, are insufficient to enable us to give a satisfactory answer to this question. It will for the present be enough to bear in mind that by some writers the environment _is_ deemed the one and sufficient cause of all the characters of living creatures. But as yet we have not even seen what _is_ the environment. Evidently physical influences--the earth, sea, or air, light, heat, and motion--do not exhaust it. One important factor would be omitted if we neglected to note the share taken in the environment of each living creature by a multitude of other living creatures which are in various ways related to it. This question must occupy us later.

But by the forms of living creatures is not meant merely their external form. Some general notion then should here at starting be obtained of their internal form--that is, of their essential structure.

The minutest and probably the simplest forms of living creatures (whether plant or animal) are such as are presented by _Bacteria_,[84] the yeast-plant and _Protoccus_. Bacteria are those minute creatures the mode of origin of which in sealed infusions has been so much of late disputed, but the activity of which in promoting the decomposition of dead substances is undisputed. A _bacterium_ is a particle of protoplasmic matter, either spheroidal or oblong, or like a short rod, or shaped like a corkscrew, and bacteria may also be in the form of a short chain of spheroids, or of oblong particles, or of rods united in a zigzag manner.

Their breadth may vary from the 1/30000 to 1/10000 of an inch. They may also assume quite another appearance, by surrounding themselves with a gelatinous envelope, which condition is called their _zooglæa_ state of existence.

They may be readily obtained by making some hay tea, and keeping it for a day or two, when they will be found to abound in the scum which forms on the surface, and to be in active motion. In the corkscrew form, _Spirillum volitans_, each end of the body is produced into a minute hair-like process or _cilium_, and it is by the lashings of these cilia that the minute organism moves about.

Other as simple but larger organisms may consist of a minute mass of semi-fluid protoplasm, containing granules, as we find to be the case in the plant _Vaucheria_,[85] and many other _Algæ_, and in the animal _Amoeba primitiva_.[86]

An organism of this simplest kind or a fragment of a higher organism which presents this simplest condition is called a cell.[87] Very generally such cell has within it a more or less distinctly marked generally denser and spheroidal body called a _nucleus_, within which, again, other minute spots may appear called _nucleoli_.

Even in this simplest of all possible conditions of life a slight difference appears between its most external film and its inner substance--just as a cup of broth left to stand will form for itself a filmy outermost layer. This incipient difference between what is inner and what is outer is one which is constantly maintained in all higher organisms, as we shall soon see abundantly. But the distinction into outer and inner is, as has been said, shown in a much more marked way in the constituent units, or _cells_, which build up the bodies of plants generally; for these consist of an inner part of protoplasm, enclosed in a distinct external cellulose envelope or _cell-wall_. As has also been shown, many of the lowest animals take on occasionally the _encysted_ condition when they also consist of a particle of bioplasm enclosed in a distinct cell-wall or _cyst_, though one not made of cellulose.

The protoplasmic contents of the cell may attract watery fluid thus forming clearer spaces or _vacuoles_ within it, and these may become so extended that the protoplasm may be reduced to a thin layer lining the cell wall, thread-like processes or remnants of protoplasm often passing across the cell from one part of the protoplasmic lining to another. A cell, almost always a nucleated cell, is the original form of every living creature without exception; and a great number of small, and some considerably sized living beings, never get beyond this unicellular condition, however much their cell may become enlarged or complicated in shape. Such creatures form the lowest of all animals and plants; but the overwhelming majority of living creatures are formed of aggregations of cells which cohere and fuse together in various ways. As an example of a unicellular and typically cellular living creature we may take the yeast plant (_Saccharomyces cerevisiæ_), which consists of a particle of bioplasm enclosed in a cell-wall of cellulose, the whole being globular or oval in shape, and generally about 1/3000 of an inch in diameter. Within its bioplasm a clear space or vacuole may often be distinguished. Often these organisms appear with a more complicated outline, due to the growth of new saccharomycetes from its outer wall, and the budding forth of others again from the side of such protruding processes, all of which ultimately become detached as independent saccharomycetes, though they often continue adherent for a long time, forming strings or other temporary aggregations of such organisms.

In _Protococcus_ we meet with one of the lowest order. Its colour is green, which, as in all other higher plants also, is due to the presence in its protoplasm of a colouring matter called _chlorophyll_, either diffused or aggregated in certain denser granules of protoplasmic substance. Protococcus may be smaller or much larger than the yeast plant, it is spheroidal, and its protoplasm is enclosed in a tough case of cellulose, which, however, it may not nearly fill, while the long cilia may protrude through it and propel the whole organism by their reiterated lashings.

It has been already said that a vegetable may temporarily exist as a particle of bioplasm without any cell-wall, and such is the case with _Protococcus_, the cellular envelope of which occasionally disappears. More remarkable still is the form already referred to under the name _Myxomycetes_,[88] which, for part of its existence, is the form of an indefinitely-shaped, naked protoplasmic mass.[89]

Living creatures which consist of a single cell may present, nevertheless, a considerable complication of structure. Thus, an organism as simple as the _amoeba primitiva_, before noticed, may have the power of forming, or, as it is technically called, _secreting_, from its own substance and its surrounding medium a most complex supporting skeleton of calcareous or silicious nature. It may have its outer envelope so markedly differentiated from its inner as to require a distinct designation as _exosarc_, while it may give rise in its interior not only to a nucleus and nucleolus, but to two regularly formed cavities with the power of rythmical pulsation, and one definite portion of its external wall may be perforated to form a permanent mouth instead of as in such forms as _Amoeba_, any part serving indifferently as a mouth and every portion having similar functions without differentiation. All these and other complications of structure may arise by direct growth and transubstantiation of the single cell into the various physically and chemically different parts.

Again, a living creature which is fixed may so extend itself as to simulate stem, roots, and branches, and yet remain essentially simple, consisting merely of one greatly enlarged and complicated cell.

Thus, a unicellular plant may take on a great complexity of form while still remaining purely unicellular. It may assume the form of a stem with roots and leaves. An example of such we may see in the genus _Caulerpa_,[90] which, although unicellular, simulates in its outline the fern called _Blechnum_.

The next grade of structural complication in living creatures is produced by the lowly plants, such as _Protococcus_, which multiply by spontaneous self-division or _fission_. This process may take place repeatedly and at the same time incompletely, in this way producing an apparently compound organism. Thus, we have the second grade of structural complication in living creatures--namely, the aggregation of cells into a loosely joined mass.

Other simple forms are those presented by the minute organisms Diatoms and Desmids, the former enclosed in silicious cases, and some presenting the only exception to the general law that organic bodies are bounded by curved lines and surfaces.

Wonderful is the minute ornamentation presented by the surfaces of these microscopic plants. Some of them cohere by imperfect division in the second grade of structural complication just described; they may form longitudinal series of cells, or they may be arranged round a common centre.

One of the best examples of this secondary grade of complication is presented by the spherically aggregated cells of _Volvox_.[91] These present us with a good example of the way in which the shape of the individual cells may spontaneously alter, to suit the mode of their aggregation. Originally spherical, the adjacent sides of these cells become flattened, and thus the cells acquire a polygonal figure.

Other instances of the coherence of the cells of unicellular organisms into indefinite and inconstant aggregations is presented by some radiolarians, individuals which cohere into what are called _colonies_.

From such incomplete aggregation, the next step is to definite and stable aggregations, in which the life of the constituent parts is more or less plainly subservient to, and dominated by, the life of the whole. Such we find in all but the lowest _Fungi_,[92] and _Algæ_, in sponges,[93] and _Hydræ_, and also in all higher organisms. In such permanent aggregations, the dominant life of the whole is shown partly in greater constancy of external form and partly in the setting apart of separate portions of the whole, either for the nourishment of the entire creature or for the reproduction of fresh individuals, or for effecting gaseous interchange, or (in animals) for ministering to feeling and locomotion.

Thus, the overwhelming majority of living creatures are, as has been said, formed of aggregation of cells, which cohere or fuse together in various ways--and not only of aggregation of cells but of aggregation of aggregations of cells or "tissues." Each tissue is a structure formed by the aggregation, or by aggregation and metamorphoses, of certain sets of cells. Thus, every higher plant or animal is made of an inconceivable multitude of cells, together with tissues which are not cellular, but which have originated by metamorphosis of cells, and every such higher plant or animal at first consists entirely of an aggregate of plainly distinct cells; and, first of all, of one single cell only, whence its whole structure, however complex, has originally sprung, though generally not until it has had at least a portion of another cell mixed with it.

This transformation of cells, at first all alike, into distinct orders of cells or _tissues_, whence different organs with different functions arise, is characteristic of all living creatures above those which each consist throughout life of one cell only.

We have seen that unicellular organisms may unite into a cylindrical or spheroidal colony, as in some _Radiolaria_, or into a spheroid of closely-adjusted cells, forming one layer, as in _Volvox_. But however large or complex such aggregation may be, it never forms sets of united cells or tissues. The whole of these lower creatures, therefore, may be spoken of as unicellular organisms; as though they may consist of many cells, those cells retain their individuality. Such creatures are all the lowest animals--those called _Hypozoa_[94] or _Protozoa_, and also the lowest cryptogamic[95] plants.

All other animals and all the higher plants are multicellular. The description of one animal (which is placed as it were on the boundary between the multicellular and the unicellular division), the little parasitic worm _Dicyema_,[96] must for the present be postponed, as its significance could not yet be understood.

Before leaving the consideration of the forms of living creatures, a further distinction should be made clear--that is to say, a distinction in the nature of resemblances which may exist between various parts.

There are two different relations which may exist between a part or organ in one animal or plant, and another part or organ in another animal or plant. One of these relations is called _analogy_ and the other _homology_, and it is very desirable to bear clearly in mind the distinction which exists between these two relations.

_Analogy_ refers to the use to which any part or organ is put--that is, it refers to its function.

Thus, the flower of the daisy is, as we shall see, analogous to that of the buttercup. The spathe of an arum is analogous to the corolla of the dead nettle (for both serve to shelter the essential parts of the flower).

The foot of a horse is analogous to the foot of a man, and the shell of a tortoise to the shell of an armadillo; for the two former serve for support and locomotion, while the latter two are solid protecting envelopes to the body. So also the flying organ or wing of a bat is analogous to the flying organ or wing of a beetle.

_Homology_ refers to essential similarity in position compared with all the other parts or organs of the body, and must be considered apart from function.

Thus, as we shall see in the next Essay a single floret of the daisy is homologous with the whole flower of the buttercup. The spathe of an arum is the homologue of any bract,[97] however insignificant in size and apparently devoid of function. The foot of a horse is homologous (as we shall see later) to the middle toe only of man, while the shell of the tortoise is in part homologous with the shell of the armadillo and in part with the ribs of the latter animal.

There is no relation of homology, however remote, between the wings of a bat and of a beetle, and these two animals (as will shortly appear) have the parts and organs of their bodies so fundamentally different, that it is doubtful whether any definite relations of homology can be established between them.

A special term has been devoted to signify a resemblance between two parts in two different animals and plants, which resemblance has been induced by or is directly related to their common needs, and the similarity of external influences. This term is "homoplasy," and structures which may thus be supposed to have grown alike in obedience to the influence of similar external causes acting on similar innate powers have been called _Homoplasts_.

Such, then, are the more general conditions as to structure and figure which living creatures present, and (as has been said) with great differences as to the amount of possible variation, most kinds have a definite limit as to size. It remains only to make general observations on the colours of living creatures.

But a few years ago, hardly any few general remarks of really scientific interest and value could have been made respecting the varied hues and markings which organisms present. No rational relation was even suspected to exist between the colours of plants and the busy insect life which swarms about their blossoms or about the varied colours of birds, and the details of their habits and modes of existence.

It was known, of course, that Arctic foxes and hares became white in winter, and that each benefited by its change, and suffered from the change of the other; the snow tint which enabled the hare to escape also facilitating the unobserved approach of the fox. It was also known that many desert animals were of the colour of the sandy plain they wandered over, and that tree-snakes and tree-frogs were often green. But it seemed incredible that the varied shades or bright adornments of the living world should each and all be governed by rigid laws, generally connected with the welfare of the organisms so furnished. Here, if anywhere, the reign of utilitarianism in Nature appeared to be at an end, and creative fancy to have full play, regardless but of the harmony and beauty thus revealed to appreciating eyes. The labours and fruitful thoughts of Bates and Wallace have, however, opened up a wide field for most interesting inquiry. They have made it evident that in many instances the most direct utility accompanies colour both in animals and plants. The colours of flowers serve to attract insects and birds, by the visits of which they are fertilized or their fertility is greatly augmented. It is this relation between attractiveness and insect fertilization which explains the absence of colour from the flowers of plants which are fertilized only by the wind, such as the fir trees before-mentioned, oaks, beeches, nettles, sedges, and many others. It also explains the conspicuousness of the flowers of many oceanic islands, such as those of the Galapagos archipelago. But it also explains, as Mr. Wallace has pointed out, the remarkable beauty of Alpine flowers, by their need of attracting insects from a distance, the conspicuous patches of bright colour serving thus to attract wandering butterflies upwards from the valleys.

But more remarkable still is the explanation given to the semblance borne by the colours of some creatures to those of others of quite a different kind, as of some moths to bees, and some harmless flies to wasps. For now it is clear that by this mimicry they escape the attacks of many enemies, who avoid such apparently dangerous forms. On the other hand, the bright liveries of such offensive creatures are highly useful to the wearers, for such tints act as a warning to enemies, and so save them from their being pounced on by creatures which might fatally wound them, though unable to swallow them. But the beautiful liveries of such powerful predatory kinds as tigers and leopards do not serve as warnings. They serve their wearers, however, none the less, though it is by aiding their concealment, and so allowing their prey to approach them unsuspectingly to fatal nearness. For the vertical stripes of the tiger resemble the vertical shadows of the grasses of the jungle amongst which it lurks, as the scattered spots of the leopard agree with the scattered spots of shadow amongst the foliage of trees on the boughs of which it lies in wait. But to say more on this head would be to anticipate remarks to come, when the relations of living beings to one another are under consideration, and the subject is too extensive to be here treated in full. Moreover, it must be noted that such relations do not by any means serve to explain all the phenomena of organic colour. Direct action is in some curious way exerted upon many organisms, by surrounding tints, and similarly different geographical districts and varieties of locality affect directly the colour of both animals and plants, but these questions will be fully treated of under the head of the relations of animals to the physical world. Suffice it here to note that the phenomena of colour no less than the phenomena of form are in harmony with (whether or not the result of) the active agencies of all environing conditions. But colour of some kind is a universal attribute of all material things. Though apparently most irregularly distributed through the world of life, yet order underlies the seeming confusion. Of certain large groups certain tints are characteristic, as has already been remarked with respect to the great order to which the dandelion belongs. But the same remark may be made of various others, as, for example, of the order _Cruciferæ_ (to which the wallflower and turnip belong), the flowers of which are generally white, pink, or yellow, while the gentians, again, are noteworthy for exhibiting pure colours.

But the colours which predominate in the whole mass of living creatures of all kinds are tints of green, brown, or reddish-yellow. Bright colours, such as blue, scarlet, crimson, gold, or silver are exceptional, and the colour blue is especially rare. The borrowed radiance of the inorganic world, in the form of metallic brightness, is especially a characteristic of those living gems, the humming birds; but not a few other animals also exhibit it. Thus, of birds more or less gifted with metallic radiance, though in a less degree than humming birds, may be mentioned the sunbirds, the trogons, and the beautiful family of pheasants; and many insects and many fishes shine with metallic tints.

Brightness of this kind (though the leaves of a few plants have a coppery lustre) is unknown in the world of plants, in which shades of green are overwhelmingly predominant, and are universally present, except in a few exceptional forms, notably the fungi.[98]