THE FORMS AND COLOURS OF LIVING CREATURES.
In the Essay on Animals and Plants, which appeared in the September Number of this Review, the names were given of the principal groups in which the prodigious multitude of living creatures (existing or known to have existed) have been classified by naturalists. It was therein also indicated that these various groups, and all the subdivisions of such groups, are distinguished one from another by variations in the forms and structures of the creatures which compose them. This fact alone would prove that very many differences in form must exist; but, indeed, a very slight knowledge and a very cursory examination of animals and plants would suffice to show this even to any one who knew nothing of the scope or nature of biological classification. In truth, to the non-scientific observer who feels an interest in living things, the difficulty may seem to be rather how to find general resemblances than how to detect differences between creatures which seem so totally diverse as do humming birds from whales, bees from buffaloes, or the numerous African herds of antelopes from the grasses on which they feed.
Nevertheless it was pointed out in the second Essay of this series[56] that all living creatures do agree to a certain extent in the form and structure of their bodies, inasmuch as their bodies are always bounded by curved lines and surfaces, while, if we divide the body of any animal or plant its structure may always be seen to be heterogeneous—that is to say, composed of different substances, even the simplest showing a variety of minute particles (granules) variously distributed throughout its interior. It has also been pointed out[57] that all living creatures agree in beginning life in the form of a small rounded mass of protoplasm. But all animals and plants further agree in that each kind has its own proper size, shape, structure, and colour, and each (as we shall hereafter see) shows a positive unity in its fundamental constitution, co-existing with the heterogeneity above referred to.
But though each kind has its own proper size, shape, structure, and colour, yet these vary more or less in different individuals, and the degrees of variability are different in different kinds both of animals and plants.
As to size, although most living creatures have certain limits which they rarely exceed or fall below, yet many organisms vary greatly in this respect. Thus, that familiar weed, the common centaury (Erythræa centaurium), may vary in height—according to the soil and other external conditions—from half an inch to five feet.
As to figure and structure there is more constancy, and the amount of variation which may in these respects be found between different individuals of the same animal species, is generally but slight. In plants and in plant-like animals much greater differences exist as to external configuration; but even in them the internal structure of each species varies but little.
Colour is a character which some readers may be disposed to regard as extremely inconstant. We are familiar with many differently coloured varieties of our cultivated flowers; and white blackbirds, and black leopards are not very uncommon objects. Nevertheless, colour is really a character of much constancy, and is one not only constantly present in different individuals of one kind of plant or animal, but is one constantly present in particular groups of kinds.
Thus, for example, all the English plants of the dandelion order which have opposite leaves, have yellow flowers, with the single exception of the eupatory (Eupatorium cannabinum), and whole groups of butterflies are respectively characterized as being blue, or white, or yellow.
We have seen that the life of every living being is accompanied by, and may be described as, a series of adjustments of action and structure to external conditions which surround it. Accordingly we may expect to find that the sizes, shapes, structures, and colours of living beings bear relations, which are in very many cases obvious, to their external circumstances, as directly favouring their nutrition, reproduction, or preservation from external injury.
Every living creature must be either fixed (like a rooted tree), or capable of spontaneously moving, or of being passively drifted from place to place, and must have a structure and figure suitable to one or other of these conditions.
Again, every living creature, whether free or fixed, is either a terrestrial, an aquatic, or an aërial organism; and it may be fitted to live in any two, or even in all three of these conditions—as, for example, is the swan. If terrestrial, it may inhabit the surface of the earth only, or it may occasionally or habitually dwell beneath it. The structure, forms, and even colours of organisms are in most cases plainly adapted to their modes of life in the above respects.
Thus, any living creature, which is fixed to the surface of the earth, must either adhere to it by having one side or portion of its body spread out and adjusted to irregularities in the supporting surface, or else by sending prolongations of its substance into the substance of the supporting body, as a plant sends its roots into the soil. Such prolongations, moreover, must (in order to hold fast) either sink deeply or else expand, at a slight depth, into a rounded or discoidal mass, or into radiating processes whereby the whole structure may be securely anchored.
This special modification of form, again, may or may not be accompanied by certain further modifications of structure, according as such rooting parts are to serve, as mere holdfasts, simply for attachment, or (as in most plants) for the absorption of food also.
Another modification is also correlated with these conditions. We have seen[58] that an interchange of gases takes place between each organism and its surrounding medium. But such interchange cannot take place in the subterranean part of the body, and a corresponding difference of structure between such subterranean part and other parts must therefore obtain.
Again, as to colour, we find differences which are evidently related to the different degrees in which different parts of a living body are exposed to the influence of light. Such contrasts notoriously exist, not only between the green parts of plants above the soil and the lighter coloured roots, but between the foliage of a plant which is exposed to sun light and another of the same kind kept in a dark cellar. Many animals which live in permanent darkness are colourless, as, e.g., the Proteus;[59] but yet this is not an invariable rule, some, as the mole, being of a dark colour.
The forms of organisms are evidently often directly related to surrounding influences. A plant or plant-like animal fixed to the soil may be so fixed that light, air, food, friends and enemies can have access equally on all sides or not. Thus, a tree so placed that light and air are excluded on one side, will not grow freely towards that side, but only in directions from whence light and air have access. A coral reef increases much more rapidly towards the open sea (the waves of which bring in food and facilitate gaseous interchange) than towards an adjacent shore.
The mere contiguity of parts will often affect the form of organisms. Thus, in many flowers parts which are adjacent become dwarfed, while others which are freely exposed become fully developed, as we see in the flowers of many Umbelliferæ, or plants of the parsley, fennel, and hemlock order.
The shapes of flowers bear relation (as we shall see later) to their need for attracting insects which by their visits effect the development of seed, and for repelling others the access of which would be hurtful.
The avoidance of enemies may be so effected by an organism that their access may be made impossible save in one direction, the extent of vulnerable surface even in that direction being minimized. We have an example of such a condition in those worms which live in calcareous tubes, and which are some of those called “tubicolous annelids.”[60]
Again, the medium in which an organism lives—whether aërial or aqueous—has an important relation with its form. A delicate seaweed, the beautifully radiating form of which is a just object of admiration as long as it is supported by its denser natural medium (the sea water), collapses into an amorphous mass when withdrawn thence into the thin air. Obviously a much greater rigidity and strength of structure is needed to support an aërial organism than an aquatic one, unless the former can support itself on other solid structures, such as rocks or trees. In the latter case the form attained may be very elongated and slender, as in the many creeping and climbing plants, which are so often furnished with processes for grasping (tendrils) to aid them in their mode of life.
An aërial fixed organism, if it does not rise from the surface of the earth, cannot spread itself very far without developing other points of support—without rooting again. This re-rooting is a familiar phenomenon in many plants, as, e.g., the strawberry. But even a shrub like the common bramble (which is not itself prostrate, but which sends out extraordinarily prolonged branches) is aided by such a process. The ends of its long branches apply themselves to the ground and begin to pierce its surface, the incipient leaves of its terminal bud becoming metamorphosed into roots.
An aquatic fixed organism, however, may extend to a very great length, freely floating without effecting any such fresh attachment. Thus the seaweed Laminaria digitata[61] will spread over a circle 12 feet in diameter, while L. longicornis grows in the form of an elongated riband, from 8 to 12 feet in length and 2 or 3 feet wide. The giant form Macrocystis (with a much more subdivided outline) may extend to the extraordinary length of 700 feet.
The conditions under which needful gaseous interchange can be effected and food obtained by different living creatures, govern in various other ways the forms of their bodies.
Thus, if it is helpful to the life of a creature to submit as large a surface of its body as possible to the influence of light, or to the action of air or water, then for this purpose its body must be expanded and its expanded parts divided and subdivided as they extend in different directions. It is for this reason that trees branch, and that their branches and twigs divide and subdivide as they do. It is for this reason also that their branches do not grow out one above another in precisely the same direction, but, on the contrary, grow in such a manner that each one may overshadow those immediately beneath as little as may be. Similarly and for the same reason leaves are developed mostly in an alternating fashion, so that each may be able to expose its green surface to the light and air as much as possible.
Plant-like animals which grow up in an arborescent manner from a fixed base do not generally branch in so regularly alternating a mode as do plants, and in some cases their successive branches may even be regularly superimposed. This is due to their not requiring, as plants do, that their surface should be very extensively exposed to light, neither their gaseous interchange nor their nutrition being impaired by such superposition. The water which carries to them both the nutritious particles on which they feed and the gases they respire, will act with nearly or quite the same efficiency in either arrangement of their parts.
If the exigences of life require any organism to retain much fluid within it, this circumstance may lead to its assumption of a dilated more or less globular form, as in the melon cactus, and, to a less degree, in the leaves of the common stonecrop.
But the conditions under which alone certain fixed organisms can obtain their food may govern also their internal structure. Thus, we shall see that in plants which feed by absorbing matters through their roots, an internal arrangement has to be effected for distributing material thus obtained, and conveying it upwards through the stem. So, again, many fixed animals need a greater supply of food and gases than they can obtain from the water which bathes or may reach them without effort on their parts. Such animals may be provided with special internal structures, which cause currents of water to flow towards them, and very often to penetrate within them, as in the shell Mya or the razor shell.[62]
Fixed subterranean creatures are rare, but such do exist, as, for example, the truffle (Tuber cibarium). Surrounding influences must in such instances be alike on all sides, while the imbedded position of such organisms render superfluous the development of any elongated process for the purpose of fixing them. Such creatures, then, have a spheroidal figure, and neither internally nor externally are their structures developed in special directions.[63]
The fixed organisms which are the most aërial in their habits are attached to elevated objects, such as trees, and necessarily have a portion of their frame set apart to fix them to the object which supports them. The most conspicuous creatures of this kind are, perhaps, the plants termed “Epiphytes,” on account of this habit. Amongst them may be mentioned the beautiful orchids called “air plants,” and the familiar mistletoe. Other vegetable organisms—the multitude of creeping plants—rear themselves to great heights by the aid of their more robust brothers, but they can hardly be reckoned as aërial organisms.[64]
The colours which plants display have sometimes a singular relation to the mountain elevations or geographical positions they inhabit, but these considerations will be aptly treated of in the relations borne by living creatures to physical conditions and to one another.
Living creatures which are capable of moving or being freely moved about, present us with similar but more marked differences.
Certain aquatic creatures drift passively about (borne by streams or currents) with no permanent relation between any fixed portion of their bodies and the medium which transports them. Such creatures being equally acted on on all sides by surrounding agencies might be expected (like the subterranean truffle) to exhibit a spheroidal figure, with only one kind of surface upon their whole exterior. This is just what we find to be the case in a variety of more or less minute organisms, such, e.g. as Myxastrum radians and Magosphæra planula.[65]
The former of these consists, at one stage of its existence, of a small globular mass of protoplasm, from the whole periphery of which a multitude of fine pseudopodia radiate. When about to reproduce, the creature retracts its pseudopodia, and forms around its exterior a structureless coat or cyst, an action which takes place frequently in lowly organisms, and is called their process of encystment. The contents of the cyst then divides into separate bodies, which escape by the rupture of the cyst. Each of these bodies is enclosed in a silicious case with an aperture at one end, whence its contained protoplasm issues, and, having so issued, assumes a spherical shape.
Magosphæra is another small creature which goes through a remarkable series of changes, the greater number of which exemplify the ball-like shape of body alike on all sides.
Wherever the surface of the body is covered by pseudopodia, those processes, inasmuch as they have a power of spontaneous movement, enable the creatures possessing them slightly to aid or to resist the drifting action of the water in which they float.
But a living organism may be devoid of any definite shape whatever, as in Protamœba,[66] which consists of a mere particle of protoplasm, from which irregular-shaped processes of unequal size are irregularly protruded in every direction, so that the form of the creature may be said to be quite indeterminate.
The bodies of almost all organisms have, however, more or less definite forms, which may be all classed under seven morphological categories.
(1). The simplest form of all exemplifies spherical symmetry, and is that which we have seen in the truffle, the radiolarian, the volvox, Myxastrum and Magosphæra. In this spherical form any number of axes drawn through the creature in any direction are equal.
(2). The next organic form is one in which the body sphere is more or less elongated at its poles, the latter being equal and similar. In such an organism we have one axis longer than any one of the others and central, while from this axis symmetrical radii can be drawn in all directions. This form may be said to exemplify equipolar symmetry, and such is found in some radiolarians, in some small parasites (Gregarinida),[67] and others.
(3). The next morphological category may be spoken of as unipolar symmetry. Bodies which exemplify it are like those included in the last category, save that the two poles of the body are not alike.
Instances of this symmetry are to be sought in creatures which have one end of their body fixed, or which always or mostly move with the same end of the body in front, and thus have their two extremities in more or less constantly different relations to surrounding influences.
The lowest worms and sponges may serve as examples of this symmetry in its simplest expression. As also may the curious compound tunicary called Pyrosoma.[68] In all such creatures the body does not extend out in the form of lateral prolongations.
But in many others it does send out processes on all sides, and in various directions, as in most trees and all plants which have a definite axis of growth, so that unipolar symmetry is the predominant symmetry in the vegetable kingdom.
(4). But unipolar symmetry with diverging outgrowths leads us to the next category which may be called radial symmetry. Under this head are included the forms of such creatures as possess unipolar bodies from which equal and corresponding outgrowths radiate in different directions.
We have examples of this in the starfishes, in the sea anemones, and in such plants as the melon cactus. But the outgrowths may project in only four directions, each being at right angles with the two neighbouring outgrowths. We thus get a crucial form of radiation, in which the body may be described as having one main axis (in the direction of motion) crossed by two other shorter but equal axes at right angles to it and to each other.
We have an example of this in Tetraplatia volitans,[69] an aquatic creature with an elongated body, which presents four distinguishable longitudinal surfaces, of which each opposite and corresponding pair is hardly distinguishable from one another.
(5). This form leads us directly to that kind of symmetry which is predominant in the animal kingdom and which is called bilateral symmetry. Forms of this kind exhibit four aspects which may be distinguished as right and left, dorsal and ventral. The body here presents a long axis (in the direction of motion) crossed by two shorter axes at right angles to it and to each other. Of these shorter axes, one connects the dorsal and ventral surfaces, while the other connects the lateral (right and left) surfaces, and these two axes may be, and generally are, unequal. All worms, insects, mollusks, fishes, birds, reptiles, and beasts, are examples of creatures with bilateral symmetry. The dorsal and ventral aspects of the body generally differ in correspondence with the different relations to surrounding conditions which they usually bear, as notably in snakes and creatures which glide with their bellies applied to the surface of the ground.
(6). The last kind of symmetry which here needs notice is that termed serial symmetry. In the creatures which exhibit it we have a body which is not only almost always bilaterally symmetrical but which is made up of a succession of similar parts, forming a series along its main or longitudinal axis. Insects, crabs, lobsters, and other allied forms give us examples of serial symmetry, but this is perhaps best seen in such animals as thousand legs and hundred legs—millipedes and centipedes.
Besides the fundamental distinctions which depend upon the kind of symmetry governing the form of any living being, other subordinate differences exist respectively related to the conditions under which the various activities necessary for life have to be carried on. Such activities are the needful gaseous interchange, the processes of reproduction, and the acquisition of food. Thus, the most intimate relation exists between the form of the body and the manner in which locomotion has to be effected, whether by the whole body or by processes projecting from it. If the latter, then whether by paddling or jumping; if by the whole body, then whether by lateral or vertical bendings of that body.
Thus, we see that fishes, which swim by lateral flexure of the body, have the tail expanded vertically; while in porpoises, which require vertical flexions (to come rapidly to the surface to breathe), the tail is expanded horizontally. On the other hand, creatures which swim not by either kind of body flexure, but by a paddling action only, have the tail shortened, as we see in swans and turtles. Further details of this kind will be more appropriately treated of in an Essay devoted exclusively to the consideration of the forms of animals.
There are a multitude of aquatic creatures which cannot be properly spoken of as either “fixed” or “mobile,” for they are in fact both. They are creatures which move about by the help of others, being themselves fixed to other creatures which are actively locomotive.
Thus, sea-snails, lobsters, fishes, whales, and even ships, bear about with them sometimes lowly-organized plants; but often other animals, permanently fixed to and growing parasitically upon them and having the shape of their body suited to their peculiar situation.
Often such parasites form flattened encrustations on their involuntary hosts—as is the case with the acorn shells or sessile barnacles.[70] Others have elongated bodies, which stream through the water with the motions of the creatures carrying them. We see this in confervoid growths, also in ordinary barnacles, and in certain modified crab-like creatures, such as Lerneocera.[71]
These creatures fix themselves to their movable supports by means similar to those by which other creatures secure themselves to stationary supports. Thus, some of these do so by means of expanded disks, which fit accurately to the supporting surface, while certain parasites fix themselves by means of ingrowing prolongations or root-like processes, as in the Rhizocephala.[72] Others, again, adhere by the intervention of hooks and suckers, and this is especially the case with such as fix themselves internally and live perpetually bathed (as the tape-worms[73] do) in the nutritious fluids contained within the bowels of the creatures they infest.
Terrestrial mobile organisms can, of course, only be moved by their own efforts, or by the efforts of other organisms.
The simplest terrestrial locomotion is like that of the aquatic Amœba[74] primitiva, and is performed by land Amœbæ; and the curious plant Myxomycetes[75] also moves in a substantially similar manner. This very curious organism consists of a net-work of protoplasmic threads, which spread over decaying leaves and stems. The threads exhibit streams of granules flowing within them, and they give out processes like pseudopodia, while the whole complex mass can slowly creep over a supporting surface, which it thus slowly flows over by its branching processes.
Other lowly plants propel themselves by means of a pair of filamentary protoplasmic threads, which vibrate actively, and are therefore called vibratile cilia. As an example may be mentioned the Protococcus[76] nivalis, the little spheroidal alga, which abounds on Alpine summits and in Arctic regions.
As in aquatic, so in terrestrial organisms, external form is intimately related to modes of motion. Thus, locomotion may be effected by undulations of the whole body, as often in serpents and terrestrial vermiform animals. It may, on the contrary, be effected by the action of levers projecting from the surface of the body, i.e., by limbs, and these may be multitudinous and minute, as in hundred legs and thousand legs, or few and large, as in beasts. Moreover, the motions may be movements of pulling or of pushing, or by combinations of these, or by jumps, which may be effected in various manners, the consideration of which will find a fitting place in an Essay devoted to “Motion.”
Again, terrestrial, like aquatic, organisms often involuntarily carry about with them other living creatures which have fixed themselves to their bodies. Thus, the fruits, or seeds, of many plants (as, e.g., those of the common Agrimony, Agrimonia eupatoria) are beset with hooks or bristles which readily adhere to the coats of passing animals, and so gain a greater diffusion than they could otherwise obtain. A very remarkable form of the kind is Martynia proboscidea (called Testa di Quaglia by the Italians), which has a pair of curved and pointed processes like the tusks of an elephant, which are several inches long. It is notorious for adhering to clothes, &c. Other noteworthy plants are Uncaria procumbeus, or the grapple plant of South Africa and Harpagophytum,[77] the fruit of which is provided with hooked processes. Those of Harpagophytum spread out in all directions, and are of different lengths, with sharp hooks, variously turned, so that its power of clinging is extreme. The seed, with all its processes, is so large as to fill the hand when grasped. It is said to cause the death of the lion. Having adhered to that beast’s skin, the irritation produced and the impossibility of getting it off at last induces the lion to bite it, and once in his mouth he cannot remove it, and so the animal dies miserably.
Some animals fix themselves much as these seeds of plants do. Amongst them are the parasites known as tics which fix themselves with great tenacity by the appendages of their mouths. Other parasites—like the itch insect[78] and forms allied to it—have hooked processes and stiff, hard bristles, which are at once very irritating and very adherent. Creatures are also carried about inside others, as is the case with the seeds of many plants. These are disseminated by birds which have swallowed but have not digested such seeds, and in an analogous manner the great tape-worm group becomes also widely diffused.
Moving subterranean organisms, inasmuch as they must penetrate through a dense and highly-resisting substance, must evidently either have forms which offer little resistance—reducing friction to a minimum—or must be provided with special means of penetrating such substance. Evidently the least resisting form is presented by a body much elongated, rounded, and more or less attenuated at the advancing end, which end has to effect the requisite penetration. This is the form of the earth-worm—a form which is approximated to by a variety of creatures which have not the least affinity of nature with it, but only more or less resemble it as regards its dwelling-place and mode of locomotion.
Such, for example, are the curious serpents called Typhlops,[79] and such are the legless lizards[80] (Anguis), and such, again, are the simpler vermiform animals allied to frogs, called Cæciliæ.[81]
In order to burrow quickly and easily by means of processes of the body, it is evidently a necessary condition that the earth should be rapidly removed by the powerful action of parts situated towards the body’s anterior end. The similarity of effect of similar conditions in creatures which are most widely divergent in nature is exemplified by the mole and the mole-cricket, which are each provided with a strong and broadened-out pair of anterior digging-limbs.
Living creatures may be sustained in the air for a longer or shorter time at one or another stage of their existence. The reproductive particles of the lowest forms of animals and plants are so excessively minute that they float in the air with the greatest ease, without needing any complication of structure—their spheroidal form harmonizing with the equal action upon them of influences on all sides of them. Reproductive parts which, though less minute than these, are still very small, may also be diffused by floating in the atmosphere. Such are the pollen grains of those trees which are fertilized merely by the action of the winds, such as the hazel, poplar, birch, and of lowly plants, as the grasses. It is by the wind that the pollen grains of these plants are accidentally brought into contact with the appropriate surfaces for their reception. Conspicuous in the spring of the year are the clouds of yellow dust, pollen grains, given off by fir trees, which are plants also wind-fertilized. But here we find a slight complication; for to facilitate the dispersion of such particles the outer coat of each of their pollen grains is produced into a short wing-like process on each side, and these processes help at once to sustain it in the air, and to aid its propulsion by offering more surface to the force of the aërial currents.
Very much more conspicuous are the wing-like expansions of many seeds—such, for example, as those of the maple. These expansions serve to diffuse the seeds which bear them, as do also the delicate cottony filaments which surround the seeds of a variety of plants of widely different natures and affinities, as some kinds of spider float through the air by the aid of the delicate filaments which they send forth to serve as an aërial float. Familiar to every one is the delicate little parachute-like structure of radiating filaments on the seeds of such plants as the dandelion—which seeds most children have at some time helped to diffuse by blowing.
Aërial progress by actual effort is effected by a limited group of organisms, and only in certain cases (bats, birds, and insects) does it take the form of true flight in creatures now existing. In other creatures, such as so-called flying fishes, squirrels, opossums, and the little flying dragon, the more or less prolonged aërial sustentation is effected by expansions of skin, which act as parachutes in ways be later described in detail.
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 Amœba 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 amœba 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 Amœba, 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]
Various aquatic animals belonging to very different groups agree in possessing a perfectly glass-like transparency. Amongst them are fish which live in the ocean; for example, the Teleostean[99] fish (Leptocephalus), also mollusca of all kinds, including even perfectly transparent cuttle fishes.[100] There are also glass-like crustaceans,[101] and also planarians[102] and sea anemones.[103] Plants, however, never present this character, although by it they might, as well as animals, escape being preyed upon.
Most fishes which inhabit the deep sea are of a dull black colour, though some are white, and the majority of all deep-sea animals, considered as a whole, are more or less decidedly coloured, many brightly so.[104]
Luminosity is a character of many lowly animals, and it is the presence of minute creatures possessing this character which so often causes the spray dashed from the prow of an advancing ship to appear like a shower of sparks, while glowing bodies traverse the water beneath its surface. Many insects, such as fire-flies and glow-worms, are notoriously luminous. In the vegetable world, however, this character is very rarely present, being only so in certain fungi, some of which exhibit a wonderful luminosity. Humboldt relates that he found this to be especially splendid in mines.
As like phenomena of colour characterize certain groups of living creatures, so also like phenomena of colour may characterize certain geographical regions being common to creatures of very different kinds which inhabit such regions, as we shall hereafter see. The brightest of living things, the humming birds, have their true home in the equatorial region of America, to which continent they are exclusively confined. But it is in the equatorial region of the whole earth that we find the most brilliant birds of other kinds, the most brightly coloured reptiles and fishes, the largest and many of the loveliest butterflies, moths and beetles, the most beautiful orchids, the largest of all flowers and of all clusters of flowers.
But neither the temperate, nor even the Arctic nor Antarctic climes are denied the glory of bright tints in the long days of their brief, but sometimes fervid, summer. Indeed, the golden burst of gorse and glow of heather in our temperate zone have, in their way, an unequal charm; while every here and there Arctic lands and Alpine heights exhibit beauties of colour which are hardly elsewhere presented by the field of animated nature to the eye of man.
St. George Mivart.