The keynote of the whole poem is struck in its opening lines. When Venus is addressed as Nature’s sole guide and ruler, this, from the poet’s own point of view, is not true of Nature as a whole, but it is eminently true of life, whether we identify Venus with the passion through which living things are continually regenerated, or with the pleasure which is their perpetual motive and their only good. And it is equally appropriate, equally characteristic of a consummate artist, that the interest of the work should culminate in a description of this same passion, no longer as the source of life, but as its last outcome and full flower, yet also, when pushed to excess, the illusion by which it is most utterly disappointed and undone; and that the whole should conclude with a description of death, not as exemplified in any individual tragedy, but in such havoc as was wrought by the famous plague at Athens on man and beast alike. Again, it is by the orderly sequence of vital phenomena that Lucretius proves his first great principle, the everlasting duration and changelessness of matter. If something can come out of nothing, he asks us, why is the production of all living things attached to certain conditions of place and season and parentage, according to their several kinds? Or if a decrease in the total sum of existence be possible, whence comes the inexhaustible supply of materials needed for the continual regeneration, growth, and nourishment of animal life? It is because our senses cannot detect the particles of matter by whose withdrawal visible objects gradually waste away that the existence of extremely minute atoms is assumed; and, so far, there is also a reference to inorganic bodies; but the porosity of matter is proved by the interstitial absorption of food and the searching penetration of cold; while the necessity of a vacuum is established by the ability of fish to move through the opposing stream. The generic differences supposed to exist among the atoms are inferred from the distinctions separating not only one animal species from another, but each individual from all others of the same species. The deflection of the atoms from the line of perpendicular descent is established by the existence of human free-will. So also, the analysis which distinguishes three determinate elements in the composition of the soul finds its justification in the diverse characters of animals—the fierceness of the lion, the placidity of the ox, and the timorousness of the deer—qualities arising from the preponderance of a fiery, an aërial, and a windy ingredient in the animating principle of each respectively. Finally, by another organic illustration, the atoms in general are spoken of as _semina rerum_—seeds of things.