Heraclitus (530–470 B.C.), who left wealth and its cares to live a life of poverty and study in the shade of the temple porticoes at Ephesus, turned science from astronomy to earthlier concerns. All things forever flow and change, he said; even in the stillest matter there is unseen flux and movement. Cosmic history runs in repetitious cycles, each beginning and ending in fire (here is one source of the Stoic and Christian doctrine of last judgment and hell). “Through strife,” says Heraclitus, “all things arise and pass away . . . . War is the father and king of alclass="underline" some he has made gods, and some men; some slaves, and some free.” Where there is no strife there is decay: “the mixture which is not shaken decomposes.” In this flux of change and struggle and selection, only one thing is constant, and that is law. “This order, the same for all things, no one of gods or men has made; but it always was, and is, and shall be.” Empedocles (fl. 445 B.C., in Sicily) developed to a further stage the idea of evolution.17 Organs arise not by design but by selection. Nature makes many trials and experiments with organisms, combining organs variously; where the combination meets environmental needs the organism survives and perpetuates its like; where the combination fails, the organism is weeded out; as time goes on, organisms are more and more intricately and successfully adapted to their surroundings. Finally, in Leucippus (fl. 445 B.C.) and Democritus (460–360 B.C.), master and pupil in Thracian Abdera, we get the last stage of pre-Aristotelian science—materialistic, deterministic atomism. “Everything,” said Leucippus, “is driven by necessity.” “In reality,” said Democritus, “there are only atoms and the void.” Perception is due to the expulsion of atoms from the object upon the sense organ. There is or have been or will be an infinite number of worlds; at every moment planets are colliding and dying, and new worlds are rising out of chaos by the selective aggregation of atoms of similar size and shape. There is no design; the universe is a machine.
This, in dizzy and superficial summary, is the story of Greek science before Aristotle. Its cruder items can be well forgiven when we consider the narrow circle of experimental and observational equipment within which these pioneers were compelled to work. The stagnation of Greek industry under the incubus of slavery prevented the full development of these magnificent beginnings; and the rapid complication of political life in Athens turned the Sophists and Socrates and Plato away from physical and biological research into the paths of ethical and political theory. It is one of the many glories of Aristotle that he was broad and brave enough to compass and combine these two lines of Greek thought, the physical and the moral; that going back beyond his teacher, he caught again the thread of scientific development in the pre-Socratic Greeks, carried on their work with more resolute detail and more varied observation, and brought together all the accumulated results in a magnificent body of organized science.
2. ARISTOTLE AS A NATURALIST
If we begin here chronologically, with his Physics, we shall be disappointed; for we find that this treatise is really a metaphysics, an abstruse analysis of matter, motion, space, time, infinity, cause, and other such “ultimate concepts.” One of the more lively passages is an attack on Democritus’ “void”: there can be no void or vacuum in nature, says Aristotle, for in a vacuum all bodies would fall with equal velocity; this being impossible, “the supposed void turns out to have nothing in it”—an instance at once of Aristotle’s very occasional humor, his addiction to unproved assumptions, and his tendency to disparage his predecessors in philosophy. It was the habit of our philosopher to preface his works with historical sketches of previous contributions to the subject in hand, and to add to every contribution an annihilating refutation. “Aristotle, after the Ottoman manner,” says Bacon, “thought he could not reign secure without putting all his brethren to death.”18 But to this fratricidal mania we owe much of our knowledge of pre-Socratic thought.
For reasons already given, Aristotle’s astronomy represents very little advance upon his predecessors. He rejects the view of Pythagoras that the sun is the center of our system; he prefers to give that honor to the earth. But the little treatise on meteorology is full of brilliant observations, and even its speculations strike illuminating fire. This is a cyclic world, says our philosopher: the sun forever evaporates the sea, dries up rivers and springs, and transforms at last the boundless ocean into the barest rock; while conversely the uplifted moisture, gathered into clouds, falls and renews the rivers and the seas. Everywhere change goes on, imperceptibly but effectively. Egypt is “the work of the Nile,” the product of its deposits through a thousand centuries. Here the sea encroaches upon the land, there the land reaches out timidly into the sea; new continents and new oceans rise, old oceans and old continents disappear, and all the face of the world is changed and rechanged in a great systole and diastole of growth and dissolution. Sometimes these vast effects occur suddenly, and destroy the geological and material bases of civilization and even of life; great catastrophes have periodically denuded the earth and reduced man again to his first beginnings; like Sisyphus, civilization has repeatedly neared its zenith only to fall back into barbarism and begin da capo its upward travail. Hence the almost “eternal recurrence,” in civilization after civilization, of the same inventions and discoveries, the same “dark ages” of slow economic and cultural accumulation, the same rebirths of learning and science and art. No doubt some popular myths are vague traditions surviving from earlier cultures. So the story of man runs in a dreary circle, because he is not yet master of the earth that holds him.
3. THE FOUNDATION OF BIOLOGY
As Aristotle walked wondering through his great zoological garden, he became convinced that the infinite variety of life could be arranged in a continuous series in which each link would be almost indistinguishable from the next. In all respects, whether in structure, or mode of life, or reproduction and rearing, or sensation and feeling, there are minute gradations and progressions from the lowest organisms to the highest.19 At the bottom of the scale we can scarcely divide the living from the “dead”; “nature makes so gradual a transition from the inanimate to the animate kingdom that the boundary lines which separate them are indistinct and doubtful”; and perhaps a degree of life exists even in the inorganic. Again, many species cannot with certainty be called plants or animals. And as in these lower organisms it is almost impossible at times to assign them to their proper genus and species, so similar are they; so in every order of life the continuity of gradations and differences is as remarkable as the diversity of functions and forms. But in the midst of this bewildering richness of structures certain things stand out convincingly: that life has grown steadily in complexity and in power;20 that intelligence has progressed in correlation with complexity of structure and mobility of form;21 that there has been an increasing specialization of function, and a continuous centralization of physiological control.22 Slowly life created for itself a nervous system and a brain; and mind moved resolutely on towards the mastery of its environment.