The Scale of Nature.
No one can closely examine a large number of plants and animals without perceiving real or imaginary gradations among them. The gradation, shrews, monkeys, apes, man, is not very far from a real genealogical succession, confirmed by structural and historical proofs. The gradation, fish, whale, sheep, on the other hand, though it seemed equally plausible to early speculators, is not confirmed by structure and history. In the age of Aristotle and for long afterwards the ostrich was believed to be a connecting link between birds and mammals, because it possessed, in addition to obvious bird-like features, a superficial resemblance to a camel (long neck, speed in running, desert haunts, and a rather imaginary resemblance in the toes). Sedentary, branching zoophytes were quoted as intermediate between animals and plants; corals and barnacles as intermediate between animals or plants and stones. Aristotle was convinced of the continuity of nature; his scale of being extended from inanimate objects to man, and indicated, as he thought, the effort of nature to attain perfection. Malpighi traced analogies between plants and animals, identifying the seed and egg, as many had done before him, assuming that viviparous as well as oviparous animals proceed from eggs, and comparing the growth of metals and crystals with the growth of trees and fungi. Leibnitz believed that a chain of creatures, rising by insensible steps from the lowest to the highest, was a philosophical necessity. Buffon accepted the same conclusion, and affirmed that every possible link in the chain actually exists. Pope reasoned in verse about a "vast chain of being," which reaches from God to man, and from man to nothing. The eighteenth century was filled with the sound.
Bonnet in 1745 traced the scale of nature in fuller detail than had been attempted before. He made Hydra a link between plants and animals, the snails and slugs a link between mollusca and serpents, flying fishes a link between ordinary fishes and land vertebrates, the ostrich, bat, and flying fox links between birds and mammals. Man, endowed with reason, occupies the highest rank; then we descend to the half-reasoning elephant, to birds, fishes, and insects (supposed to be guided only by instinct), and so to the shell-fish, which shade through the zoophytes into plants. The plants again descend into figured stones (fossils) and crystals. Then come the metals and demi-metals, which are specialised forms of the elemental earth. Water, air, and fire, with perhaps the æther of Leibnitz, are placed at the bottom of the scale.
In Bonnet's hands the scale of nature became an absurdity, by being traced so far and in so much detail. It was not long before a reaction set in. The great German naturalist, Pallas, in his Elenchus Zoophytorum (1766) showed that no linear scale can represent the mutual relations of organised beings; the branching tree, he said, is the appropriate metaphor. Cuvier taught that the animal kingdom consists of four great divisions which are not derived one from another, and his authority overpowered that of Lamarck, who still maintained that all animals form a single graduated scale. A complete reversal of opinion ensued, so complete that at length the theologians, who had once seen in the scale of nature a proof of the wisdom of Providence, were found fighting with all their might against the insensible gradations which, according to Darwin's Origin of Species, must have formerly connected what are now perfectly distinct forms of life.
The eighteenth-century supporters of continuity in nature were not merely wrong in picturing the organised world as a simple chain or scale. They were also wrong in assuming that all the links or steps still exist. We can now see that vast numbers are irrecoverably gone. It is a safe prophecy that the filiation of species will never be grasped by the intelligence of man except in outline, and even an outline which shall truly express the genetic relations of many chief types is unattainable at present.