and asks, What then is the original substance out of which existing things have arisen? The answer is, "Atoms and the Void, and beside them nothing else:" these two principles are solid, self-existent, indestructible, and invisible. He next investigates and refutes the first principles of other philosophers, notably Heraclitus, Empedocles, and Anaxagoras; and the book ends with a short proof that the atoms are infinite in number and space infinite in extent. The Second Book opens with a digression on the folly of ambition; but, returning to the atoms, treats of the combination which enables them to form and perpetuate the present variety of things. All change is ultimately due to the primordial motion of the atoms. This motion, naturally in a straight line, is occasionally deflected; and this deflection accounts for the many variations from exact law. Moreover, atoms differ in form, some being rough, others smooth, some round, others square, &c. They are combined in infinite ways, which combinations give rise to the so-called secondary properties of matter, colour, heat, smell, &c. Innumerable other worlds besides our own exist; this one will probably soon pass away; atoms and the void alone are eternal. In the Third Book the poet attacks what he considers the stronghold of superstition. The soul, mind, or vital principle is carefully discussed, and declared to be material, being composed, indeed, of the finest atoms, as is shown by its rapid movement, and the fact that it does not add to the weight of the body, but in no wise _sui generis_, or differing in kind from other matter. It is united with the body as the perfume with the incense, nor can they be severed without destruction to both. They are born together, grow together, and perish together. Death therefore is the end of being, and life beyond the grave is not only impossible but inconceivable. Book IV. treats of the images or idols cast off from the surface of bodies, borne continually through space, and sometimes seen by sleepers in dreams, or by sick people or others in waking visions. They are not illusions of the senses; the illusion arises from the wrong interpretation we put upon them. To these images the passion of love is traced; and with a brilliant satire on the effects of yielding to it the book closes. The Fifth Book examines the origin and formation of the solar system, which it treats not as eternal after the manner of the Stoics, but as having had a definite beginning, and as being destined to a natural and inevitable decay. He applies his principle of "Fortuitous Concurrence" to this part of his subject with signal power, but the faultiness of his method interferes with the effect of his argument. The finest part of the book, and perhaps of the whole poem, is his account of the "origin of species," and the progress of human society. His views read like a hazy forecast of the evolution doctrine. He applies his principle with great strictness; no