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 _Protamoeba_,[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
_Amoeba[74] primitiva_, and is performed by land _Amoebæ_; 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
_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]
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.