Remarkable, however, as this uniformity of result undoubtedly is, it becomes still more striking when we contemplate the variety of sources from which food is derived for the support of animal life. To use the words of an able writer already quoted, “There is no part of the organized structure of an animal or vegetable, however dense its texture or acrid its qualities, that may not, under certain circumstances, become the food of some species of insect, or contribute in some mode to the support of animal life. The more succulent parts of plants, such as the leaves or softer stems, are the principal sources of nourishment to the greater number of larger quadrupeds, to multitudes of insects, as well as to numerous tribes of other animals. Some plants are more particularly assigned as the appropriate nutriment of particular species, which would perish if these ceased to grow: thus the silk-worm subsists almost exclusively upon the leaves of the mulberry-tree; and many species of caterpillars are attached each to a particular plant, which they prefer to all others. There are at least fifty different species of insects that feed upon the common nettle; and plants of which the juices are most acrid and poisonous to the generality of animals, such as euphorbium, henbane, and nightshade, afford a wholesome and delicious food to others.”[17] Nor are the precision and accuracy with which the same fluid--the blood--affords to every structure of the body the precise species of nourishment or secretion which its elementary composition requires, however different each may be from the rest in chemical qualities, less admirable and extraordinary than its own original formation from such a variety of materials. To bone, the blood furnishes the elements of bone with unerring accuracy; to muscle the same blood furnishes the elements of muscle,--to nerves the elements of nerve,--to skin the elements of skin,--and to vessels the elements of vessels;--and yet, while each of these differs somewhat in composition from the others, the constituent elements of the blood by which they are furnished are everywhere the same. Similar phenomena, indeed, occur in the vegetable world; but this, instead of diminishing our wonder, tends rather to augment it. The same elements, extracted from the same soil, are converted into every variety of vegetable product--into leaves of every shade of green, flowers of every form and tint, and juices of every quality, from the deadly poison up to bland and life-supporting milk. Nay, even in the same plant--as in the poppy--we sometimes find the seeds and the capsule which covers them endowed with the most opposite properties.