"The colors of insects," he says, "and many smaller animals contribute to conceal them from the dangers which prey upon them. Caterpillars which feed on leaves are generally green; earthworms the color of the earth which they inhabit; butterflies, which frequent flowers, are colored like them; small birds which frequent hedges have greenish backs like the leaves, and light-colored bellies like the sky, and are hence less visible to the hawk, who passes under them or over them. Those birds which are much amongst flowers, as the goldfinch (_Fringilla carduelis_), are furnished with vivid colors. The lark, partridge, hare, are the color of dry vegetables or earth on which they rest. And frogs vary their color with the mud of the streams which they frequent; and those which live on trees are green. Fish, which are generally suspended in water, and swallows, which are generally suspended in air, have their backs the color of the distant ground, and their bellies of the sky. In the colder climates many of these become white during the existence of the snows. Hence there is apparent design in the colors of animals, whilst those of vegetables seem consequent to the other properties of the materials which possess them" (_The Loves of the Plants_, p. 38, note). In his _Zoonomia_ (§ xxxix., vi.) Darwin also speaks of the efficient cause of the various colors of the eggs of birds and of the hair and feathers of animals which are adapted to the purpose of concealment. "Thus the snake, and wild cat, and leopard are so colored as to resemble dark leaves and their light interstices" (p. 248). The eggs of hedge-birds are greenish, with dark spots; those of crows and magpies, which are seen from beneath through wicker nests, are white, with dark spots; and those of larks and partridges are russet or brown, like their nests or situations. He adds: "The final cause of their colors is easily understood, as they serve some purpose of the animal, but the efficient cause would seem almost beyond conjecture." Of all this subject of protective mimicry thus sketched out by the older Darwin, we find no hint or trace in any of Lamarck's writings. 8. Great length of time. He speaks of the "great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind" (p. 240). In this connection it may be observed that Dr. Darwin emphatically opposes the preformation views of Haller and Bonnet in these words: