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Page:Popular Science Monthly Volume 24.djvu/220

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THE POPULAR SCIENCE MONTHLY.

wanderers upon the ocean so far from their native habitat, and I find that they nearly all exhibit to a greater or less extent the same curious characteristics. Here the observant mariner with a smattering of science may find something to cogitate upon.

Light, heat, sound, etc., are said to be effects produced upon the living organism through the medium of appropriately developed organs by as many different modes of motion, whereby the animal is brought into conscious connection with surrounding objects, the effect diminishing in a progressive ratio as the distance of the object increases. Of these special organs there are said to be five in number which are essential to the well-being of all perfectly developed animals. But may there not be other analogous modes of motion, producing analogous effects upon the living organism, whereby the animal is brought into conscious connection with surrounding objects, and by or through which it has a sense of the locality or direction of such objects as are essential to its well-being to seek or avoid? Admitting this, suppose a flock of birds have started on one of their migratory excursions, guided mainly by this sense of direction, in pursuit of some distant object. Then let us suppose that in their flight they pass obliquely, but unwittingly, into another higher stratum or current of air moving with great velocity in some other direction, but toward the ocean. The flock would necessarily become very much scattered, and in the confusion a portion of them would be carried unconsciously out to sea, beyond the range of their sense of direction having lost which, they fly at random and at ease, exerting just sufficient effort to sustain themselves in the air; while another portion of the flock, keeping within the limit of their sense of direction, will exhaust all their strength vainly endeavoring to reach their object against a violent wind.

So intimately associated with this sense of direction is their instinct of self-preservation in avoiding the presence of man, that while the one is in abeyance, the other, in the absence of anything to arouse it, remains dormant. This, I believe, is the true meaning of the preternatural tameness exhibited by the birds on the Galapagos Islands mentioned by Darwin.

From the peculiar properties of air in its relation to heat, the atmosphere has a tendency to form itself into heterogeneous strata, more or less inclined to the horizon; each stratum having a horizontal motion independent of the others—a fact the significance of which, I think, is frequently lost sight of by meteorologists, more especially by cyclonologists. That some of the higher strata of the atmosphere have an independent horizontal motion, the velocity of which is often incomparably greater than anything we experience in the lower stratum, is evident not only from the appearance in mid-ocean of birds, but of insects hundreds of miles from land, and apparently as lively as if they were in their own native haunts. I have seen grasshoppers at