places for its abode with which it most corresponds in color. Dwelling on this point, he remarks that "the instinct of self-preservation native to all animals, which sharpens their senses against incessantly threatening dangers, prompts marine as well as land animals to seek dwelling-places similar to themselves in form and color." Any person who has had opportunity, as I have, to watch the little crabs and shrimps, that swim around in the alga-groups of the Sargasso Sea, for half a day at a time, will have to admit that there is much in this view, though it will not be necessary to throw away the theory of selection. I can not say whether the crab I have observed is the same that Wagner describes as Nautilo graspus minutus. I have collected several hundreds of the animals, and think, after superficial observation, that I can distinguish more than one species among them, while the variability, especially in color, is wonderful. The adaptation of the innumerable tints to every grade of change in the color of the sea-weed is really marvelous. The younger, lighter green crustaceans are always to be found on the young, verdant fronds of the plant, while the older parts of the weed are inhabited by older, brown animals. The older stems are often incrusted with the white shells of bryozoa, and corresponding with these we are sure to find white spots on the brown armor of the crabs. The legs of the animals are frequently of an olive green ground with brownish spots, deceptively like the slender seaweed-leaves that are just beginning to turn brown. If one will, as I did, pull one of the large plants upon the deck, leave it in a cask of sea-water for an hour or two, and then look through it for crabs without disturbing it, he will find it very hard to discover three or four of the animals, although he may be sure there are a quarter of a hundred of them there; and, if he gives the mass a lively shake, he will find a curious assemblage of the most varied sorts tumbling off the bush, whose behavior will go far to verify Wagner's view; for, if they are allowed the opportunity, they will all swim back to the sea-weed, and each will seek a part of the plant most like it in color. I tried the experiment forty or fifty times, and never saw a little green crab settle on a dark-brown stem. The crustaceans keep to their color, and the brown ones will, with amazing speed, dart through the thick net-work of stems and leaves, to the darkest spot they can find, where they quickly escape observation.
I remarked another striking example of what might be considered intelligent mimicry in a crab, on the 11th of September, after we had got out of the Sargasso Sea. Toward night a piece of dark-brown bark, about as large as one's hand, floated close by the ship. Thinking I might find something upon it, I fished it up with a scoop I had prepared for such purposes and which I had found very useful, and put it in a pail of fresh sea-water for observation. While I stood looking at it, I perceived motions of legs and tentacles, and then discovered a crab, so precisely of the color of the bark, that it might have lain on