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 chemica............