sands of the desert they frequent. The Polar Bear, living on ice and snow, is white or gray; but as the summer advances and the snow passes away, leaving the dark ground exposed to view, the Bear changes his skin to a brown or black, assuming again, as winter returns, its whitish hue. Mr. Darwin explains these striking facts by showing that the harmonizing of the color of an animal with its surroundings is useful to it. For those animals being unobserved are favored in the struggle for existence, seizing more easily their prey, or escaping from their enemies more readily, than those not so favored. Mr. Wallace, speaking of the butterfly Kallima parapleta, says, "At length I was fortunate enough to see the exact spot where the butterfly settled, and, though I lost sight of it for some time, I at length discovered that it was close before my eyes, but that in its position it so closely resembled a dead leaf attached to a twig as almost certainly to deceive the eye even when gazing full upon it." In reference to this subject, Prof. Haeckel notices the Helmichthys, fishes whose bodies are so transparent that one can read a book through them. The Carinaria among the Mollusca, the Salpa among the Worms, many of the Jelly-fishes, are either bluish or colorless as the water they live in. The transparent glass-like color of these animals who live on the surface of the open sea is evidently of service to them in catching the objects of their prey or avoiding their enemies. Suppose now the remote ancestor of one of these animals to have been slightly transparent, a little more so than the individuals of the same species, it would have been favored in the struggle for existence, and would have survived. Transmitting this useful peculiarity, its posterity would be still more transparent. Finally, in the course of generations, almost perfectly transparent animals would be produced. Prof. Cope observes, "The gray sand hue so well adapted for concealment is universal, with few variations, in the
