Even the lower jaw, which presents characters so peculiar as to admit, as I conceive, of but one interpretation--that enunciated in my memoir on the teeth and jaws of the Iguanodon,[138]--has been adduced as affording a signal instance of the incorrectness of my physiological deductions. And why? Because in the entire class of living reptiles there is not a single species that has cheeks and flexible lips, which, according to my view of the subject, the Iguanodon must have possessed. But I do not hesitate to affirm that the structure and arrangement of the teeth, and the mammalian character of the bones of the extremities, are in perfect accordance with my exposition of the probable structure and functions of the maxillary organs of the original. The naturalists who advance these objections, forget that among the existing mammalia there is one genus, the _Ornithorhynchus_, or Duck-billed Platypus, that exhibits as striking a deviation from the typical maxillary structure of its class, as does the Iguanodon. If before the discovery of New Holland the jaw-bones of the Ornithorhynchus had been found in a fossil state in the strata of Tilgate Forest, and I had ventured to infer that the original, though a true mammalian, and giving suck to its young, had the extremities of the jaws covered with flat horny beaks, like those of a duck, instead of with the fleshy lips and integuments which are the peculiar attributes of its class, what censures would not my temerity have called forth! We cannot too often be reminded of the profound remark of William Penn: "Experience, which is continually contradicting theory, is the only test of truth." [Footnote 138: See Philosophical Transactions, Part II. 1848.] The following are the physiological inferences relating to the structure and habits of the Iguanodon, which Dr. Melville and myself conceive our investigations have established: the discovery of the cranium, and of perfect examples of the upper and lower jaws with both successional and mature molars in their natural position, may modify, but, we believe, will in no material respect invalidate these conclusions.