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212 appearing suddenly in the air from the back of the hay-stack, had been mistaken for a hawk, and that the bird so mistaking it had immediately uttered the appropriate warning note. Unfortunately for my little mouse" ("theories," says Voltaire, "are like mice; they run through nineteen holes, but are stopped by the twentieth"), "only the other day, when I was at the same place and equally near, a genuine hawk (a sparrow one) had flown by, when, instead of a warning note, there had been a sudden hush and silence, followed by a flight which, as it seemed to me, was not so close and compact as usual. Difficulties of this sort are always occurring in observation—at least in my observation—and show how cautious one should be in translating the particular into the general. For instance, with moor-hens, I have noted that in one or two of their many timorous flights to the river a peculiar cry was uttered by a single bird, which had all the appearance and seemed to have all the probability of being a warning note; but this was not the case on other occasions." Even here, then, there is some difficulty in accepting the theory of a danger-signal uttered by one bird, and causing the simultaneous flight of all, whilst in all other instances (I am speaking now of small birds at the stacks) either no note at all or none distinguishable from a general chirping was uttered. Manifestly, then, this explanation will not serve. But it may be said, either that there is a leader whose movements all the birds follow, or that when one bird flies, for whatever reason, the rest take alarm and fly also. But where different species of birds are all banded together, it seems very