Special Adaptations.
Along such general lines, such open roads as these, animal life has progressed, and without such limitations as to prevent further progress or adaptations to new conditions. But I wish now particularly to call attention to certain adaptations that result in a definite limitation of the animal, a fitness to special conditions and a fitness so complete that existence under other conditions is impossible, or to put it still more broadly, a return adaptation is probably impossible. Such lines of adaptation may be looked upon as by-paths or blind alleys sought out by certain forms as presenting easier conditions for existence or into which feeble species may be crowded by the force of stronger ones. Places where certain shifts provide adequate chance for survival, albeit on a lowly plane.
Such by-paths are innumerable—almost as the nooks and crannies into which organisms may crowd—and to catalogue them would be to survey a large field of zoology. They are especially interesting and instructive as showing in most emphatic manner the factors that have been operative in modifying structure and attesting the general fact of evolution. The animals that are sedentary, domestic, subterranean, parasitic; the inhabitants of caves, deserts, manufactured products, oil, vinegar, hot springs, snow and ice, on islands, under bark, in deep sea, are illustrations of such erratic departures from normal habits. While we can review but few, these few may serve to illustrate the principles involved and some may be grouped under general heads.
Perhaps the least departure from normal, free-living conditions is presented by those animals which assume a sedentary habit. This may range all the way from a temporary anchorage in mud or on a rock to permanent attachment with most fundamental changes in form and structure of the organism. It is exhibited in some degree by almost every group of animals, and were it not that its tendency is toward limitation and restriction of powers we might look upon it as one of the main avenues of development. For minute forms the attached bell animalcules, stentors, etc., are good examples, and in sponges we find this habit of fixture a constant feature and associated with marked inferiority in symmetry and activity. Hydroids, and especially corals, show it strongly developed, though the former often present free living stages alternately with the fixed. Some worms and mollusks have assumed the rôle and it was the rule with echinoderms in early time, though modern forms have largely broken away from it, not, however, until their whole symmetry had been impressed by the results of their position. The barnacles among Crustacea have gone farthest in this direction, and their symmetry and structure have been so strongly influenced that it is not strange that earlier naturalists failed to suspect
