PROTECTIVE MIMICRY IN MARINE LIFE. 367 



places for its abode with which it most corresponds in color. Dwell- 

 ing 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 Wag- 

 ner 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 si^ecies among them, while the variabil- 

 ity, especially in color, is wonderful. The adaptation of the innumer- 

 able 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 corre- 

 sponding 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 sea- 

 weed-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 with- 

 out 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 ex- 

 periment 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 pre- 

 pared 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 



