“Yet had not the Greeks pushed their abstract speculations so far, had not Kepler and Galileo made the above-mentioned discoveries, we never could have seen the greatest work that ever came from the hands of man, Sir Isaac Newton’s Principia. “Some obscure person, whose name is not so much as known, diverting himself idly, as a stander-by would have thought, with trying experiments on a seemingly contemptible piece of stone, found out a guide for mariners on the ocean, and such a guide as no science, however subtil and sublime its speculations may be, however wonderful its conclusions, would ever have arrived at. It was mere curiosity that put Sir Thomas Millington upon examining the minute parts of flowers; but his discoveries have produced the most perfect and most useful system of botany that the world has yet seen. “Other instances might be produced to prove, that bare curiosity in one age, is the source of the greatest utility in another; and what has frequently been said of chemists, may be applied to every other kind of vertuosi. They hunt, perhaps, after chimeras and impossibilities; they find something really valuable by the bye. We are but instruments under the Supreme Director, and do not so much as know, in many cases, what is of most importance for us to search after; but we may be sure of one thing, viz. that if we study and follow nature, whatever paths we are led into, we shall at last arrive at something valuable to ourselves and others, but of what kind we must be content to remain ignorant.” CHAP. VI. A GENERAL VIEW OF THE INTERNAL PARTS OF INSECTS, AND MORE PARTICULARLY OF THE CATERPILLAR OF THE PHALÆNA COSSUS---A DESCRIPTION OF SUNDRY MISCELLANEOUS OBJECTS. The interior part of insects includes four principal viscera; the spinal marrow, the intestinal bag, the heart, and tracheal vessels. The spinal marrow, or principal trunk of the nerves of insects, is a whitish thread, extended the whole length from the head to the hindermost part, furnished at intervals with small knots or ganglions. From these knots proceed the nervous threads that are supposed to be the instruments of sensation and motion. On the medullary thread is placed the intestinal bag, which is equal to it in length; it is a long gut, in which are contained the oesophagus, the stomach, and intestines. Along the back, and parallel to the intestinal bag, runs a long thin vessel, in which may be perceived through the skin of the insect alternate contractions and dilatations; this part is supposed to perform the functions of the heart. The tracheal vessels of insects are very similar to those of plants; are of the same structure, colour, and elasticity, and are, like them, dispersed through the whole body.