PRIMITIVE POLARITIES—PLANT AND ANIMAL.
Contrast in developed life—Plants producers,
animals consumers—Differences disappear in simple
forms—Zoophytes—Protista—Nummulites—Corals—Fungi—Lichens—Insectivorous
plants—Geological succession—Primary period, Algæ and
Ferns—Secondary period, Gymnosperms—Tertiary and recent,
Angiosperms—Monocotyledons and Dicotyledons—Parallel evolution
of animal life—Primary, protista, mollusca, and fish—Secondary,
reptiles—Tertiary and recent, mammals.
Animals or plants? Judging by first impressions, nothing can be more
distinct. No one, whether scientific or unscientific, could mistake
an oak tree for an ox. To the unscientific observer the tree differs
in having no power of free movement, and apparently no sensation or
consciousness; in fact, hardly any of the attributes of life. The
scientific observer sees still more fundamental differences, in the
fact that the plant feeds on inorganic ingredients, out of which it
manufactures living matter, or protoplasm; while the animal can only
provide itself with protoplasm from that already manufactured by the
plant. The ox, who lives on grass, could not live on what the grass
thrives on, viz. carbon, oxygen, hydrogen, and nitrogen. The contrast is
so striking that the vegetable world has been called the producer, and
the animal world the consumer, of nature.
Again, the plant derives the material framework of its structure from
the air, by breathing in through its leaves the carbonic dioxide present
in the atmosphere, decomposing it, fixing the carbon in its roots, stem,
and branches, and exhaling the oxygen. The animal exactly reverses the
process, inhaling the oxygen of the air, combining it with the carbon
of its food, and exhaling carbonic dioxide. Thus, a complete polarity
is established, as we see in the aquarium, where plant and animal
life balance each other, and the opposites live and thrive, where the
existence of either would be impossible without the other.
Sharp, however, as the contrast appears to be in the more specialised and
developed specimens of the two worlds, we have here another instance of
the difficulty of trusting to first impressions, and have to modify our
conceptions greatly, if we trace animal and vegetable life up to their
simplest forms and earliest origins. In the first place, each individual
vegetable or animal begins its existence from a simple piece of pure
protoplasm. This develops in the same way into a nucleated cell, by whose
repeated subdivision the raw material is provided for both structures
alike. The chief difference at this early stage is that the animal cells
remain soft and naked, while those of vegetables secrete a comparatively
solid cell-wall, which makes them less mobile and plastic. This gives
greater rigidity to the frame and tissues of the plant, and prevents the
development of the finer organs of sensation and other vital processes,
which characterise the animal. But this is a difference of development
only, and the origination of the future life from the speck of protoplasm
is the same in both worlds.
If, instead of looking at the origin of individuals, we trace back the
various forms of animal and vegetable life from the more complex to the
simpler forms, we find the distinctions between the two disappearing,
until at last we arrive at a vanishing point where it is impossible to
say whether the organism is an animal or a plant.
A whole family, comprising sponges, corals, and jelly-fish, are called
Zoophytes, or plant-animals, from the difficulty of assigning them to one
kingdom or the other. On the whole they rather more resemble animals,
and are generally classed with them, though they lack many of their
most essential qualities, and in form often bear a close resemblance to
plants. But when we descend a step lower in the scale of existence we
come to a large family—the Protista—of which it is impossible to say
that they are either plants or animals. In fact, scientific observers
have classed them sometimes as belonging to one and sometimes to the
other kingdom; and it was an organism of this class, looking at which
through a microscope Huxley pronounced it to be probably a plant, while
Tyndall exclaimed that he would as soon call a sheep a vegetable. They
are mostly microscopic, and are the first step in organised development
from the Monera, which are mere specks of homogeneous protoplasm. Small
as they are they have played an important part in the formation of the
earth’s crust, for the little slimy mass of aggregated cells has in
many instances the power of secreting a solid skeleton, or a minute and
delicate envelope or shell, the petrified remains of which form entire
mountains. Thus the nummulitic limestone, which forms high ranges on the
Alps and Himalayas, and of which the Pyramids are built, consists of
the petrified skeletons of a species of Radiolaria, or many-chambered
shells, forming the complicated and elegant mansion with many rooms
and passages, of the formless, slimy mass which constitutes the living
organism. Chalk also, and the chalk-like formation which is accumulating
at the bottom of deep oceans, are the results of the long-continued fall
of the microscopic snowdrift of shells of the Globigenera and other
protistic forms swimming in the sea; and in a higher stage of development
the skeletons of corals, one of the family of Zoophytes or plant-animals,
form the coral reefs and islands so numerous in the Pacific and Indian
Oceans, and are the basis of the vast masses of coralline limestone
deposited in the coal era and other past geological periods.
As development proceeds the distinction between plants and animals
becomes more apparent, though even here the simplest and earliest
forms often show signs of a common origin by interchanging some of the
fundamental attributes of the two kingdoms. Thus, the essential condition
of plant existence is to live on inorganic food, which they manufacture
into protoplasm, by working up simple combinations into others more
complicated. Their diet consists of water, carbonic dioxide, and ammonia;
they take in carbonic dioxide and give out oxygen, while animals do
exactly the reverse. But the fungi live, like animals, upon organic food
consisting of complicated combinations of carbon, which they assimilate;
and, like animals, they inhale oxygen and give out carbonic dioxide.
Lichens afford a very curious instance of the association of vegetable
and animal functions in the same plant. They are really formed of two
distinct organisms: a body which is a low form of Alga or sea-weed,
and a parasitic form of fungus, which lives upon it. The former has a
plant life, living on inorganic matter and forming the green cells, or
chlorophyll, which are the essential property of plants, enabling them
under the action of the sun’s rays to decompose carbonic dioxide; while
the parasite lives like an animal on the formed protoplasm of the parent
stem, forming threads of colourless cells which envelop and interlace
with the original lichen of which they constitute the principal mass, as
in a tree overgrown with ivy.
Even in existing and highly developed plants we find some curious
instances of reversion towards animal life. Certain plants, for instance,
like the Dionæa or Venus’ fly-trap, finding it difficult to obtain the
requisite supply of nitrogenous food in a fluid state from the arid or
marshy soil in which they grow, have acquired a habit of supplying the
deficiency by taking to an animal diet and eating flies. Conjoined with
this is a more highly developed sensitiveness, and power of what appears
to be voluntary motion, and a faculty of secreting a sort of gastric
juice in which the flies are digested. The fundamental property also
of decomposing carbonic dioxide and exhaling oxygen depends on light
stimulating a peculiar chemical action of the chlorophyll, and at night
leaves breathe like lungs, exhaling not oxygen, but the carbonic dioxide.
The records of geology, imperfect as they are, show a continued
progression from these simple and neutral organisms to higher and more
differentiated forms, both in the animal and vegetable worlds. These
records are imperfect because the soft bodies of the simpler and for the
most part microscopic forms of protoplasm and cell life are not capable
of being preserved in petrifactions, and it is only when they happen to
have secreted shells or skeletons that we have a chance of identifying
them. Still we have a sufficient number of remains in the different
geological strata to enable us to trace development. Thus, in the
vegetable world, in the earliest strata, the Laurentian, Cambrian, and
Silurian, forming the primordial period, which forms a thickness of some
70,000 feet of the earth’s crust—or more than that of the whole of the
subsequent strata, Primary, Secondary, Tertiary, and Quaternary, taken
together—we find only vegetable remains of the lowest group of plants,
that of the Tangles or Algæ, which live in water. Forests of these
sea-weeds, like those of the Aleutian Islands, in some of which single
tangles stream to the length of sixty feet, and floating masses, like
those of the Sargasso Sea, appear to have constituted the sole vegetation
of these primæval periods.
The Primary epoch, which comes next, comprises the Devonian or Old Red
Sandstone, the Carboniferous or Coal system, and the Permian, the average
thickness of the three together amounting to about 42,000 feet. In these
the family of Ferns predominates, the remains of which constitute the
bulk of the large strata of coal, forming in modern times our great
resource for obtaining the energy which, in a transformed shape, does so
much of our work. Pines begin to appear, though sparingly, in this epoch.
The Secondary epoch comprises the Triassic, the Jurassic, and the
Cretaceous or Chalk formation, the average thickness of the three
amounting to about 15,000 feet. In this era a higher species of
vegetation predominates, that of the Gymnosperms, or plants having naked
seeds, of which the pines, or Coniferæ, and the palm-ferns, or Cycadeæ,
are the two principal classes. As in the case of the former epoch, traces
of the approaching higher organisation in the form of leaf-bearing trees
began to appear towards its close.
The Tertiary period extends from the end of the Chalk to the commencement
of the Quaternary or modern period. It is divided into the Eocene or
older, the Miocene or middle, and the Pliocene or newest Tertiary system;
though the division is somewhat arbitrary, depending on the number of
existing species, mostly of shellfish, which have been found in each.
The average thickness of the three together is about 3,000 feet. In this
formation a still higher class of vegetation of the same order as that
now existing, which made its first appearance in the Chalk period, has
become predominant. It is that of Angiosperms, or plants with covered
seeds, forming leafy forests of true trees. This group is divided
into the two classes of monocotyledons or single-seed-lobed plants,
and dicotyledons or plants with double seed-lobes. The monocotyledons
spring from a single germ leaf, and are of simpler organisation than the
other class. They comprise the grasses, rushes, lilies, irids, orchids,
sea-grasses, and a number of aquatic plants, and in their highest form
develop into the tree-like families of the palms and bananas.
The dicotyledons include all forms of leaf-bearing forest trees, almost
all fruits and flowers, in fact by far the greater part of the vegetable
world familiar to man, as coming into immediate relation with it, except
in the case of the cultivated plants, which are developments of the
monocotyledon grasses.
We see, therefore, in the geological record a confirmation of the
evolution over immense periods of time of the more complex and perfect
from the simple and primitive.
If we turn to the same geological record to trace the development of
animal life, we find it running a parallel course with that of plants.
The earliest known fossil, the Eozoon Canadiense, from the Lower
Laurentian, is that of the chambered shell of a protista of the class
of Rhizopods, whose soft body consists of mere protoplasm which has not
yet differentiated into cells. As we ascend the scale of the primordial
era, traces of marine life of the lower organisms begin to appear, until
in the Silurian they become very abundant, consisting however mainly
of mollusca and crustacea, and in the Upper Silurian we find the first
traces of fishes.
In the Primary era the Devonian and Permian formations are characterised
by a great abundance of fishes, of the antique type, which has no true
bony skeleton, but is clothed in an armour of enamelled scales, and whose
tail, instead of being bi-lobed or forked, has one lobe only—a type of
which the sturgeon and garpike are the nearest surviving representatives.
In the Coal formation are found the first remains of land animals in the
form of insects and a scorpion, and a few traces of vertebrate amphibious
animals and reptiles; while higher up in the Permian are found a few more
highly developed reptiles, some of which approximate to the existing
crocodile. Still fishes greatly predominate, so that the whole Primary
period may be called the age of fishes, as truly as, looking at its
flora, it may be called the age of ferns.
In the Secondary period reptiles predominate, and are developed into a
great variety of strange and colossal forms. The first birds appear,
being obviously developed from some of the forms of flying lizards, and
having many reptilian characters. Mammals also put in a first feeble
appearance, in the form of small, marsupial, insectivorous creatures.
In the Tertiary period the class of mammals greatly predominates over
all other vertebrate animals, and we can see the principal types slowly
developing and differentiating into those at present existing. The
human type appears plainly in the middle Miocene, in the form of a
large anthropoid ape, the Dryopithecus, and undoubted human remains are
found in the beginning of the Quaternary, if not, as many distinguished
geologists believe, in the Pliocene and even in the Miocene ages.
So far, therefore, there seems to be a complete parallelism between the
evolution of animal and vegetable life from the earliest to the latest,
and from the simplest to the most complex forms. These facts point
strongly to a process of evolution by which the animal and vegetable
worlds, starting from a common origin in protoplasm, the lowest and
simplest form of living matter, have gradually advanced step by step,
along diverging lines, until we have at last arrived at the sharp
antithesis of the ox and the oak tree. It is clear, however, that this
evolution has gone on under what I have called the generalised law of
polarity, by which contrasts are produced of apparently opposite and
antagonistic qualities, which however are indispensable for each other’s
existence. Thus animals could not exist without plants to work up the
crude inorganic materials into the complex and mobile molecules of
protoplasm, which are alone suited for assimilation by the more delicate
and complex organisation of animal life. Plants, on the other hand,
could not exist without a supply of the carbonic dioxide, which is their
principal food, and which animals are continually pouring into the air
from the combustion of their carbonised food in oxygen, which supplies
them with heat and energy. Thus nature is one huge aquarium, in which
animal and vegetable life balance each other by their contrasted and
supplemental action, and, as in the inorganic world, harmonious existence
becomes possible by this due balance of opposing factors.