We are accustomed perhaps to think of degeneration as necessarily implying a disadvantage in life. It is true that a blind, footless, degenerate animal could not cope with the active, keen-sighted, highly organized non-degenerate in free competition. But free competition is exactly what the degenerate animal has nothing to do with. Certainly the _Sacculina_ and the scale-insects live well; they are admirably adapted to the kind of life they lead. A parasite enjoys certain obvious advantages in life, and even extreme degeneration is no drawback (except as we shall see later), but gives it a body which demands less food and care. As long as the host is successful in eluding its enemies and avoiding accident and injury the parasite is safe. Its life is easy as long as the host lives. But the disadvantages of parasitism and degeneration are nevertheless obvious. The fate of the parasite is bound up with the fate of the host. "When the enemy of the host crab prevails, the _Sacculina_ goes down without a chance to struggle in its own defence. But far more important than the disadvantage in such particular or individual cases is the fact that the parasite cannot adapt itself in any considerable degree to new conditions. It has become so modified, so specialized to adapt itself to the very special conditions under which it now lives, it has gone so far in giving up organs and functions, that if present conditions change and new ones come to exist the parasite cannot adapt itself to them. The independent free-living animal holds itself, one may say, able and ready to adapt itself to any new conditions of life. The parasite has risked everything for the sake of a sure and easy life under the present existing conditions. Change of conditions means its extinction." [Illustration: FIG. 161.--Young fur seals, _Callorhinus ursinus_, of the Tolstoi rookery, St. Paul Island, Bering Sea, killed by a parasitic intestinal worm, _Uncinaria_ sp. (Photograph by the Fur Seal Commission.)] For an elementary account of commensalism and parasitism see Jordan and Kellogg's "Animal Life," pp. 172-200. The account here given is based on the author's previously written account in "Animal Life." See also Van Beneden's "Animal Parasites and Messmates." CHAPTER XXXI COLOR AND PROTECTIVE RESEMBLANCES