And, after all, even now man is by no means such a master of the kingdoms of life as he is apt to imagine. The sea, that mysterious nursery of living things, is for all practical purposes beyond his control. The low-water mark is his limit. Beyond that he may do a little with seine and dredge, murder a few million herrings a year as they come in to spawn, butcher his fellow air-breather, the whale, or haul now and then an unlucky king-crab or strange sea-urchin out of the deep water, in the name of science; but the life of the sea as a whole knows him not, plays out its slow drama of change and development unheeding him, and may in the end, in mere idle sport, throw up some new terrestrial denizens, some new competitor for space to live in and food to live upon, that will sweep him and all his little contrivances out of existence, as certainly and inevitably as he has swept away auk, bison, and dodo during the last two hundred years. For instance, there are the Crustacea. As a group the crabs and lobsters are confined below the high-water mark. But experiments in air-breathing are no doubt in progress in this group--we already have tropical land-crabs--and as far as we know there is no reason why in the future these creatures should not increase in size and terrestrial capacity. In the past we have the evidence of the fossil _Paradoxides_ that creatures of this kind may at least attain a length of six feet, and, considering their intense pugnacity, a crab of such dimensions would be as formidable a creature as one could well imagine. And their amphibious capacity would give them an advantage against us such as at present is only to be found in the case of the alligator or crocodile. If we imagine a shark that could raid out upon the land, or a tiger that could take refuge in the sea, we should have a fair suggestion of what a terrible monster a large predatory crab might prove. And so far as zoological science goes we must, at least, admit that such a creature is an evolutionary possibility.