Page:Popular Science Monthly Volume 62.djvu/350

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By Professor WESLEY MILLS, M.A.. M.D.,


IN the course of years I have had the opportunity of observing a considerable number of animals totally blind in either one or both eyes and to these only shall I refer in this paper.

Pigeons.—All those observed, that were otherwise normal, have been blind on one side only. The defect does not seem to have resulted in any great inconvenience or disadvantage to the individual. Birds do not, it is likely, possess binocular vision, in the sense in which the term may be applied to many mammals; and in consequence of the defect, cutting off the field of vision on one side completely, the bird endeavors to make up for this by adaptive movements of the head, which it can bring about with a facility not possible to the mammal.

It also, through experience, becomes more alert than the average pigeon; nevertheless, when the struggle for existence becomes keen, as for example when a limited quantity of food is strewn about, it is shown to be plainly at a disadvantage, both in securing the food and in the bodily conflicts that are apt to arise between it and its competitors.

White Rats.—I have observed several white rats, some of which were blind in only one eye, others in both. The results were in some respects very different from what might have been expected, and in this the difference between the bird and the rabbit, on the one hand, and the rat, on the other, was striking.

Even in the case of total blindness, the rat is not handicapped as one might suppose must be the case. In a very few days the rat blind in only one eye seems to ordinary observation to be in no appreciable degree worse off than his fellows. In a short time the specimen, totally blind, moves about so well that one would need to look carefully to be assured that he gets no assistance from the visual sense. But in his case there are times when it is evident that he is handicapped. In exploring new surroundings he proceeds with special caution, stretching out his neck, sometimes resting on his hind legs, more frequently elevating his fore parts and sniffing in an unusual way, showing an extreme care and plainly making use of his acquired greater facility, or perhaps one should say, his accustomed and habitual greater use of senses that normally are not required to function to such a marked degree.

He is also somewhat more timid and retreats in the way rats do toward their place of exit, with greater readiness. He is of course more