ANTMAL LIFE. S^l 



Special Adaptatiojis. 



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 liv- 

 ing stages alternately with the fixed. Some worms and mollusks have 

 assumed the role 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 influ- 

 enced that it is not strange that earlier naturalists failed to suspect 



