PRINCIPLES OF VERTEBRATE MORPHOLOGY 19 



Ant-eating adaptations. Ants for a long time have been extremely 

 numerous and have been an important factor in the environment of 

 all land vertebrates since the establishment of the reptilian orders. 

 Wheeler in his classic treatise on ants claims that many vertebrate 

 integumentary structures, such as hair, feathers, and scales are pri- 

 marily for protection against ants. Whether this position is justi- 

 fied or not, it is certainly true that ant-eaters among all of the classes 

 of land vertebrates are heavily armored or densely covered with in- 

 tegumentary structures. Besides this most obvious adaptation, ant- 

 eaters have long slender snout, with small terminal mouth, long 

 sticky tongue, teeth reduced or absent, strong digging feet, and in- 

 ternal nostrils far forward and in such relation to the glottis that 

 ants could not possibly crawl into the wind-pipe. 



2. The law of divergence of form; the law of adaptive radiation. 

 This law is the antithesis of the "Law of Convergence"; for, in- 

 stead of a similarity of adaptive character acquired by unrelated 

 groups, we have diversity of adaptive characters acquired by members 

 of a single related stock, which tend to radiate into all of the avail- 

 able life zones and to develop the various adaptive complexes. For 

 example, a primitive stock of cursorial reptiles splits up into fossorial,. 

 aquatic, arboreal, volant, and giant herbivorous and carnivorous 

 types. A type once arboreal may become a volant type, and that 

 seems to be the usual sequence. A volant type does not become 

 fossorial, but may return to a cursorial habit, as for example, the 

 running birds. An aquatic type may become terrestrial and then 

 secondarily return to the life in the water, carrying back with it, 

 however, an air-breathing mechanism. When the aquatic verte- 

 brates developed adaptations for land life they lost their gills, their 

 lateral-line organs, etc. When once a character is lost it cannot be 

 regained, so the aquatic reptiles and mammals must be dependent 

 on lungs, although they would be much better off with gills. This 

 illustrates the irreversibility of evolutionary changes, and especially 

 of adaptive specializations. Osborn, in his book on "The Origin 

 and Evolution of Life," takes the position that the irreversibility 

 of evolution is due to the progressive chemical evolution of 

 the "heredity chromatin." Its changes, he believes, are orderly and 

 progress step by step toward more and more narrow specializa- 

 tion. Once the chromatin has acquired factors for specialized 

 characters, it cannot reverse and return to the generalized condi- 



