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, alg? 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[93] 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[94] 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[95] 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,[96] 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 dion?a 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,[97] 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 alg?, 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 prim?val 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 conifer?, and the[98] palm-ferns, or cycade?, 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[99] 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[100] 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[101] 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.
