In like manner all the essential processes by which life lives, moves, and has its being, are equally mechanical and chemical. If the brain, receiving a telegram from without through the optic nerve, sends a reply along another nerve which liberates energy stored up in a muscle and produces motion, the messages are received and transmitted like those sent by a voltaic battery along the wires of a telegraph, and the energy is stored up by the slow combustion of food in oxygen, just as that of the steam-engine is produced by the combustion of coal. All this is mechanical, inorganic, and therefore polar. But when we come to the conditions of life proper, we find the influence of polarity mainly in this: that as it develops from simpler into more complex forms, it does so under the law of developing contrasts or opposite polarities, which are necessary complements of each other’s existence. Thus, as we ascend in the scale of life, we find two primitive polarities developed: that of plant and animal, and that of male and female. CHAPTER VII. 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.