Nos. 455-456.] MAMMALIA. 817 



the arboreal and the aerial, the terrestrial habitat being sub- 

 ordinate because the upland Flora was largely undeveloped or 

 inedible as compared with its present condition. The three 

 available provinces were occupied by reptiles, mammals and 

 birds respectively. In the later Cretaceous the spread of a 

 great and varied upland flora vastly extended the terrestrial 

 province, and opened a new and constantly 



expansion 



il These then 



great 



in the Ter- 



2w terrestrial groups expandmg con- 

 ....... and becoming adapted to various modes of life. The 



arboreal types maintained or increased their lead in intelligence, 

 but changed comparatively little in other respects. The terres- 

 trial types became far more numerous and dominant, ^^lapting 

 their primitively arboreal organization to their various mo es 

 of life, yet retaining, in spite of extensive changes, a certain 

 fixity of type which had been impressed upon them by their 

 long arboreal residence. 



The great extension of the terrestrial prov 

 tiary may be supposed to have opened a correspondingly large 

 field for the expansion of the birds, but these, retaimng m the 

 main their aerial life, suffered but little change m organization, 

 and the vast majority are today as homogeneous in skeleton 

 structure as the mammals were at the beginning of the 1 er- 

 tiary. Coincident with the expansion of the terrestria pro\- 

 ince was a great contraction of the amphibious-aquatic prov- 

 ince. The extensive swamps and deltas and great inland seas 

 of the Cretaceous, shrank at its close to small prop(>rtKWs. 

 and the Reptilia underwent a corresponding 

 groups completely disappearing, others surviving ^ 

 Tertiary in continually decreasing numbers, the 

 (lizards and snakes) alone prospering. Both ^ ' 

 tiles, so far as they have retained their typical habitats, n. 

 changed but little structurally since the Mesozoic ; on 

 few terrestrial reptiles and terrestrial or aquatic b'^'^' ' 

 amount of change comparable with that in the terrestiia _ 

 maha. It would seem therefore that all the facts accord uith 

 the explanation of the evolution of mammals durmg the Cenc. 



, , , . . • o new orov nee and change 



zoic as caused by their invasion ot a new pi 

 of habitat from arboreal to dominantly terrestrial. 



