But this faculty of his understanding is in general so united with the imaginative feeling through which he discerns the vital indentity of the most diverse manifestations of some common principle, that it can best be illustrated in connexion with the poetical, as distinct from the logical, merits of the work. So also it is difficult to separate his faculty of clear, exact, and vivid observation from his poetical perception of the life and beauty of Nature. His powers of observation were, however, stimulated and directed by scientific as well as poetic interest in phenomena. From the wide scope of his philosophy he was led to examine the greatest variety of facts, physical as well as moral. His sense of the immensity of the universe led him to contemplate the largest and widest operations of Nature,--such as the movements of the heavenly bodies, the recurrence of the seasons, the forces of great storms, volcanoes, etc.; while, again, the theory of the invisible atoms drew his attention to the minutest processes of Nature, in so far as they can be perceived or inferred without the appliances of modern science. Thus, for instance, in a long passage beginning-- Denique fluctifrago suspensae in litore vestes,[405] he shows by an accumulation of instances that there are many invisible bodies, the existence of which is inferred from visible effects. In other places he draws attention to the class of facts which have been the basis of the modern science of geology,--such as the mark of rivers slowly wearing away their banks,--of walls on the sea-shore mouldering from the long-continued effects of the exhalations from the sea,--of the fall of great rocks from the mountains under the wear and tear of ages. Again, the argument is frequently illustrated by observation of the habits of various animals. In these passages Lucretius shows the curiosity of a naturalist, as well as the sympathetic feeling and insight of a poet. How graphic, for instance, is his description of dogs following up the scent of their game-- Errant saepe canes itaque et vestigia quaerunt.[406] How happily their characteristics are struck off in the line-- At levisomna canum fido cum pectore corda.[407] The various cries and habits of birds are often observed and described, as-- Et validis cycni torrentibus ex Heliconis Cum liquidam tollunt lugubri voce querellam;[408] and again-- Parvus ut est cycni melior canor ille gruum quam Clamor in aetheriis dispersus nubibus austri.[409] The description of sea-birds, Mergique marinis Fluctibus in salso victum vitamque petentes,[410] recalls the vivid and natural life of those that haunted the isle of Calypso-- τανύγλωσσοί τε κορῶναι εἰνάλιαι τῇσίν τε θαλάσσια ἔργα μέμηλεν.[411]