Many of us in our schooldays have admired the intelligence of Jackdaws having their nests in some old tower or belfry. They are able to distinguish according to the hour the significance of the various school bells. Most of these clangs do not move them, and they continue to attend to their affairs without paying attention. Their attention is only attracted by the ringing which marks the beginning and the end of recreation time. At the sound of the first they all flee and abandon the courts before even a single pupil has yet appeared. The bell, on the contrary, which marks the end of recreation time invites them to descend in a band to collect the crumbs of lunch. They arrive in a hurry, so as to be the first to profit by the repast, not waiting even until the place is abandoned; they know very well that the young people still there are not to be feared, having no time now to be occupied with them. In this class of facts, there are a certain number which may be considered as more marked by custom and perhaps less marked by spontaneous reflection. Such, for example, is the custom of Sharks and Seagulls to follow ships. In the seas where Dog-fish are abundant, one or more of them become attached to a ship, and quit it neither night nor day. One may believe sometimes that they are not there; but if any object is thrown into the sea, the fin of one of these monsters appears at the surface; everything which is thrown overboard disappears in their large jaws--kitchen refuse, bottles, etc. When a dead body is thrown into the sea it is soon seized by the shark, while living men who fall into the water have great difficulty in escaping, and are often drawn up horribly mutilated and half dead. Sea-gulls also follow vessels when they approach the coast. It is a pleasant sight to see the noisy band animating the monotonous splendour of the ocean; they arrive as soon as a vessel is one or two days' journey from land. Henceforth they do not leave her, flying behind and plunging in her wake; they profit by the disturbance produced by the gigantic machine to capture the stunned fishes. On land exactly the same kind of chase is carried on by Rooks, Crows, and Magpies, who follow the plough to seize the worms which the ploughshare turns up in the open earth. In autumn they cover the fields, animated and active, pilfering as the furrow is hollowed out.