The remarkable thing is that Nature permits the community to be taxed in this way apparently without protest. For the parasite is a consumer pure and simple. And the "Perfect Economy of Nature" is surely for once at fault when it encourages species numbered by thousands which produce nothing for their own or for the general good, but live, and live luxuriously, at the expense of others? Now when we look into the matter, we very soon perceive that instead of secretly countenancing this ingenious device by which parasitic animals and plants evade the great law of the Struggle for Life, Nature sets her face most sternly against it. And, instead of allowing the transgressors to slip through her fingers, as one might at first suppose, she visits upon them the most severe and terrible penalties. The parasite, she argues, not only injures itself, but wrongs others. It disobeys the fundamental law of its own being, and taxes the innocent to contribute to its disgrace. So that if Nature is just, if Nature has an avenging hand, if she holds one vial of wrath more full and bitter than another, it shall surely be poured out upon those who are guilty of this double sin. Let us see what form this punishment takes. Observant visitors to the sea-side, or let us say to an aquarium, are familiar with those curious little creatures known as Hermit-crabs. The peculiarity of the Hermits is that they take up their abode in the cast-off shell of some other animal, not unusually the whelk; and here, like Diogenes in his tub, the creature lives a solitary, but by no means an inactive life. The _Pagurus_, however, is not a parasite. And yet although in no sense of the word a parasite, this way of inhabiting throughout life a house built by another animal approaches so closely the parasitic habit, that we shall find it instructive as a preliminary illustration, to consider the effect of this free-house policy on the occupant. There is no doubt, to begin with, that, as has been already indicated, the habit is an acquired one. In its general anatomy the Hermit is essentially a crab. Now the crab is an animal which, from the nature of its environment, has to lead a somewhat rough and perilous life. Its days are spent among jagged rocks and boulders. Dashed about by every wave, attacked on every side by monsters of the deep, the crustacean has to protect itself by developing a strong and serviceable coat of mail.