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Show of the bird, it has to be recorded that these animals have otoliths considerably larger than those of mammals with the exception of the Seal as noted before. They are two in number and are almost in contact. One is a fiat plate, and lies in the upper and posterior portion of the vestibule ; the second is round and appears to lie on the first. Without going into the physiology of the vestibule and canals as ascertained by laboratory experiments, I would like to make a few remarks on the bearing which these structures may have upon the migrations of animals. We know, from clinical and experimental evidence, that the semicircular canals furnish the individual with an accurate knowledge of the extent to which the head has been rotated in space, whether this has been carried out by a voluntary effort on the part of the individual or by some external agency. Now, bearing this fact in mind, it has long since occurred to me that by this means we may, in part, be able to account for that mystery which has long puzzled the naturalist: the facidty by which many animals pursue their long migrations. Almost all birds migrate, a great many fishes, and even some mammals such as the Seal. That they cannot guide themselves by the sense of sight entirely is obvious, since they may pursue their flight undeterred by the darkest night and through blinding fog. Furthermore, it is difficult to understand how fishes and seals can obtain much information by vision; and, indeed, it is clear that some other sense must be employed, though vision may undoubtedly help. The same difficulty occurs in the case of the Corn-Crake, which appears to do its migration on foot, and vision can hardly avail it very much on its journey. But the sense of direction may be obtained by other means than by the eye, namely by the semicircular canals and the vestibule. When a particular canal of one side is injured, the animal tends to rotate in a particular direction, thus indicating that the canal in question regulates the movements of the animal in that direction. Of course this explanation does not account for the orienting process which the bird must go through before it starts its flight, but only for the faculty it possesses of pursuing the course correctly through the night or fog without having recourse to the sense of sight. How the orienting process is carried out, I do not pretend to explain, though it may be done pei'haps by observing the position of the setting sun or by the bearing of surrounding objects. Few mammals migrate in the proper sense of the term, and even those which do appear to do so, go from place to place in quest of food. The Seal, however, is one example of a mammal which does migrate in the strict meaning of the word, returning to a circumscribed breeding-place every year in a manner similar to the bird and the fish. Now it is rather remarkable, that of all mammals the vestibule 144 THE MEMBRANOUS LABYRINTH OF CERTAIN ANIMALS. [Mar. 7, |
