Degeneration can be produced by other causes than parasitism. It is evident that if for any other reason an animal should adopt an inactive fixed life it would degenerate. The barnacles (see fig. [37]) are excellent examples of degeneration through quiescence. They are crustaceans related most nearly to the crabs and shrimps. The young barnacle just from the egg is a six-legged, free-swimming larva (nauplius) with a single eye, greatly like a young prawn or crab. It develops during its independent life two compound eyes and two large antennæ. But soon it attaches itself to some stone or shell, or pile or ship's bottom, giving up its power of locomotion, and its further development is a degeneration. It loses its compound eyes and antennæ, and acquires a protecting shell. Its swimming feet become modified into grasping organs, and it loses most of its outward resemblance to the typical members of its class. The Tunicata or ascidians compose a whole group of animals which are fixed in their adult condition and have thus become degenerate. They have been likened to a "mere rooted bag with a double neck." In their young stage they are free-swimming, active, tadpole-like or fish-like larvæ, possessing organs much like those of the adult simplest fish or fish-like animals. Their larval structure reveals, however, the relationships of the ascidians to the vertebrates, a relationship which is not at all apparent in the degenerate adults. Certain insects live sedentary or fixed lives. All the members of one large family, the Coccidæ, or scale-insects (figs. [62] and [63]), have females which as adults are wingless and in some cases have no legs, eyes, or antennæ, while the males are all winged and have legs and the special sense organs. The males lead a free active life, but the females have nearly or quite given up the power of locomotion, attaching themselves by means of their sucking beak to some plant, where they obtain a sufficient food-supply (plant-sap) and lay their eggs. In both males and females the larvæ are little active crawling six-legged creatures with legs, eyes, and antennæ.

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."

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