…inexplicable manner in which he now depresses his head altogether beneath the surface, and anon swims with it high elevated out of the water; considering the unobstructed elasticity of its envelope; considering the unique interior of his head; it has hypothetically occurred to me, I say, that those mystical lung-celled honeycombs there may possibly have some hitherto unknown and unsuspected connexion with the outer air, so as to be susceptible to atmospheric distension and contraction.
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susceptible to colds
Not being susceptible to contradicting evidence, it is not a testable hypothesis.