Because the bird obtains his food by _skimming along the water_, into which he dips his bill, and lifts his food out. 1136. _Why do the mandibles of the cross-bill overlap each other?_ Because the bird requires a peculiar bill, to enable it to _split seeds into halves_, and to tear the open cones of the fir-tree. 1137. _Why are the tails of fishes so much larger than their fins?_ Because their tails are their _chief instruments of motion_, while their fins are employed simply to direct their progress, and steady their movements. 1138. _Why have oxen, and other quadrupeds a tough ligament called the "pax-wax," running from their backs to their heads?_ Because their heads are of considerable weight; and having frequent occasion to lift them, they are provided with an elastic ligament, which is fastened at the middle of their backs, while its other extremity is attached to the head. This enables them to raise their heads easily; otherwise the effort to do so would be a work of great labour. To the horse, the pax-wax acts as a natural bearing-rein, assisting it to hold its head in that position which adds to the grace and beauty of the animal. In carving beef, this ligament may be seen passing along the vertebræ of the neck, the chuck, and the fore ribs. [Verse: "He shall feed his flock like a shepherd; he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young."--ISAIAH XL.] 1139. _Why have the females of the kangaroo and opossum tribes pouches, or pockets, formed in the skin of their breasts for the reception of their young?_ Because their young ones are remarkably _small and helpless_; in fact, more so than those of any other animal of equal proportions. Besides which, the full grown animals have very long hind-legs, and they progress by a series of extraordinary leaps. It would consequently be impossible for their helpless young ones to follow them: God has therefore given to female kangaroos and opossums curious pockets, _formed out of their own skin_, in which they place their little young ones, and bear them through their surprising leaps with the greatest ease and safety. CHAPTER LVIII. 1140. _What is the difference between an animal, a plant, and a mineral?_ The great naturalist, Linnæus, used to say that animals _grow_, _live_, and _feel_; plants _grow_ and _live_; and minerals _grow_. Animals are here defined to enjoy _three_ conditions of existence; plants _two_ conditions; and minerals _one_ condition. This definition has, in latter days, been held to be unsatisfactory, since there _are a few plants_ that are _supposed to feel_, and _a few animals_ that are supposed to have even _less feeling_ than the _sensitive plants_ alluded to. The concise definition by Linnæus, nevertheless, is true, as far as regards a _vast majority_ of the bodies constituting the three great kingdoms of nature. And it may be sufficient to say that