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 I02 Bird -Lore departure were incomplete. Before entering upon the theory it might be as well to inquire a little into the practical working of orientation. The act of orientation is extremely difficult to observe. When an animal goes astray we know generally where he has been lost ; but we do not know, very often, by what way he has reached that particular place. If we let loose a flock of Carrier Doves we soon lose sight of them, and we only find them again in the Dove cote. Between the point of departure and the point of arrival there is a lacune for the observer. It is this gap we think we have suc- ceeded in filling. Basing ourselves on laws purely hypothetical at the beginning, we have succeeded in reconstituting the complete itinerary of animals closely observed, to follow them, in a way, step by step. We have equally studied the fault of instinct, the error of orientation, and we have verified that the puzzled animal obeys the rules, confirming the laws which we are going to formulate. We had at our disposal a grand field for observation. M. le Ministre de la Guerre gave us the task of constructing a movable Pigeon cote, which represents the practical illustration of our theor}'. Finally, La Compagnie Transatlantique requested us to organize a Carrier Pigeon post to be utilized for the service of their steamships. To the numerous experiences on land and sea we have added very interesting observations, of which the results have been communicated to us by credible witnesses. To sum up, we bring a great number of facts, man_v of which, controlled by the ofBcial reports of the commissioners representing the Minister of War, have the character of veritable discoveries. We have grouped the acts of orientation in two categories : near orientation, attributed to the exercise of the five senses, showing observation at work and, in a certain sense, reasoning and intelli- gence ; and distant orientation, an act purely mechanical, accomplished by means of a subjective sense to which we give the name of the Sense of Direction. In each of these two cases the mechanism of orientation obeys distinct laws. In the study of mathematics we often employ a method which consists in considering as proved a proposition presented as a problem and in drawing a deduction from it. We will do the same. Let us admit as a hypothetical law that the instinct of orientation is a faculty that all animals possess in a greater or less degree, of resuming the reverse scent of a road once crossed by them, and then let us use it to explain certain facts not explicable in any other way.