is no more than an hypothesis, and will probably for ever remain one. Now, an hypothesis, although indispensable as a provisional mode of grouping together facts, and giving them some sort of explanation, is after all only a _guess_, and it may be absurdly wide of the truth. In Natural History, as in all other departments of speculative ingenuity, there have been a goodly number of outrageously extravagant hypotheses, gravely propounded, and credulously accepted. Men prefer an absurd guess to a blank; they would rather have a false opinion than no opinion; and one of the last developments of philosophic culture, is the power of _abstaining_ from forming an opinion, where the necessary data are absent. If you wish to see how easily hypotheses are formed and accepted, you need only turn over the history of any science. If you want a laugh at credulity, read a chapter of Pliny’s _Natural History_. Pliny is a classic, and was for centuries an authority; but looked at with impartial eyes, he appears the veriest “old woman” that ever wrote in a beautiful style. He was a mere bookworm, without a particle of scientific insight. His was not an age when men had much regard to evidence; but to him the suspicion never seems to have occurred that Gossip Report could be given to romancing, or that travellers could “see strange things.” No fable is too monstrous for his credulity. One of the pretty fables Pliny repeats, is, that pearls are formed by drops of dew falling into the gaping valves of the oyster. It never occurred to him to ask whether oysters were ever exposed to the dew? whether the drops _could_ fall into their valves? whether oysters kept their valves open, except when under water? or, finally, whether, if the dew _did_ fall in, it would _remain_ a rounded drop? The drop of dew had a certain superficial resemblance to the pearl, and that was enough. Ælian’s hypothesis was somewhat better: he supposed that the pearls were produced by lightning flashing into the open shells. Turning from these ancient sages, you will ask how pearls are formed? And almost any ingenious modern, not a zoologist, will tell you (and tell you falsely), that the pearl is a disease of the oyster. One is somewhat fatigued with the merciless frequency with which this notion has been dragged in, as an illustration of genius issuing out of sorrow and adversity; and it is time to stop that “damnable iteration” by discrediting the notion. Know then, that if “Most wretched men Are cradled into poetry by wrong: They learn in suffering what they teach in song”—