36 THE PALEONTOLOGIC RECORD 



that radiation of different parts of the body is not necessarily cor- 

 related ; that is, that the adaptive divergence of the feet and limbs may 

 take one direction, while that of the teeth and skull may take another 

 direction. Thus great variety in combinations of characters may arise, 

 bringing about the very antithesis of Cuvier's supposed "law of cor- 

 relation " ; for we find that while the end results of adaptation are such 

 'that all parts of an animal conspire to make the whole adaptive, there 

 is no fixed correlation either in the form or rate of development of 

 parts, and that it is, therefore, impossible for the paleontologist to 

 predict the anatomy of an unknown animal from one of its parts only, 

 unless the animal happen to belong to a type generally familiar. For 

 example, among the land vertebrates the feet, which are associated with 

 the structure of the limbs and trunk, may take one of many lines of 

 adaptation to different media or habitats, either aquatic, terrestrial, 

 arboreal or aerial; while the teeth, which are associated with the struc- 

 ture of the skull and jaws, also may take one of many lines of adapta- 

 tion to different kinds of food or modes of feeding, whether herbivorous, 

 insectivorous or carnivorous. Through this independent adaptation of 

 different parts of animals to their specific ends there have arisen among 

 vertebrates almost unlimited numbers of combinations of food and tooth 

 structure. 



Alternations of Habitat. In the long vicissitudes of time and 

 procession of continental changes animals have been subjected to alter- 

 nations of habitat either through their own migrations or through the 

 "migration of the environment itself," to employ Van den Broeck's 

 epigrammatic description of the profound and sometimes sudden en- 

 vironmental changes which may take place in a single locality. The 

 traces of alternations of anatomical adaptation corresponding with 

 these alternations of habitat are recorded both in paleontology and 

 anatomy. For example, Huxley in 1880 briefly suggested the arboreal 

 origin of all the marsupials, a suggestion which has been confirmed 

 abundantly by the detailed studies of Dollo and Bensley, according to 

 which we may imagine that the marsupials have passed through a series 

 of phases, as follows: (1) a very early "terrestrial or ambulatory" 

 phase, (2) a "primary arboreal" phase as exemplified by the tree 

 phalangers of the present day, (3) a "secondary terrestrial" phase 

 as exemplified by the kangaroos and wallabies, (4) a " secondary ar- 

 boreal " phase as exemplified by the tree kangaroos. 



Each one of these phases has left its anatomical record in the struc- 

 ture of the feet and limbs, although this record is often obscured by 

 adaptation. 



Louis Dollo especially has contributed most brilliant discussions of 

 this theory of "alternations of habitat" as applied not only to the 

 interpretation of the anatomy of the marsupials but of many kinds of 



