The beasts of the forest retire to the thickets; the birds hide themselves beneath the foliage of the trees, or in the crevices of the rocks. Yet, amid this apparent silence, when we lend an attentive ear to the most feeble sounds transmitted by the air, we hear a dull vibration, a continual murmur, a hum of insects, that fill…the lower strata of the air. Nothing is better fitted to make man feel the extent and power of organic life. Myriad insects creep upon the soil, and flutter round the plants parched by the ardour of the Sun. A confused noise issues from every bush, from the decayed trunks of trees, from the clefts of the rock, and from the ground undermined by lizards, millipedes and [caecilians]. There are so many voices proclaiming to us, that all nature breathes; and that, under a thousand different forms, life is diffused throughout the cracked and dusty soil, as well as in the bosom of the waters, and in the air that circulates around us.from the Personal Narrative of Alexander von Humboldt, published 1819-29.
In The Invention of Nature, Andrea Wulf links this to the final paragraph of Darwin's Origin
A short account of Humboldt's influence on Darwin and an online edition of the Narrative are here.
Picture via here