THE GEOLOGIC FUNCTIONS OF LIFE. 671 



This variation profoundly affects the development of the fauna. 

 When a major deformation of the earth takes place which increases the 

 capacity of the oceanic basins, the water is drawn down into them more 

 fully, and correspondingly retreats from the continental shelf. The 

 shore is thus carried out toward or to the border of the shelf, or even 

 perhaps down to some line on the abysmal slope. In either case, the 

 zone of shallow water suited to the photobathic hfe is narrowed, and 

 at points it may be practically cut in two. There are, however, shelves 

 and tracts that were below the light zone before, which now are brought 

 within it by the lov;ering of the sea-level. Into these, as into harbors 

 of refuge, the life migrates so far as it may. But these tracts are less 

 prevalent and continuous than the typical continental shelf, and under 

 the conditions supposed they would be but imperfectly connected with 

 each other by available shallow-water tracts. (The steep shelving 

 shore tracts, although furnishing a shallow-water connection possibly 

 available for some species, would be unsuited to others and, under cer- 

 tain conditions of the sea- currents, would be an effective barrier.) To 

 these limited tracts, therefore, the life of the photobathic type is 

 restricted and measurably isolated, and develops into local and provin- 

 cial faunas. 



After a deforming movement has ceased, the seashore habitually 

 advances, developing a new continental shelf, and in time new epiconti- 

 nental gulfs and seas. In this it is assisted by the erosion of the conti- 

 nent and the filling of the sea, and probably by the slow settling of the 

 continents. As the sea-shelf broadens, the isolated tracts, the harbors 

 of refuge, become connected, and migration is facihtated. When the 

 connection becomes general and broad, and when epicontinental seas 

 have formed available tracts across the face of the continents, a general 

 commingling of faunas follows, and a cosmopolitan fauna results. 



In the same way, but more obviously, when the land is extended 

 and connection between the continents becomes general, there is migra- 

 tion and commingling of the land faunas and floras, and cosmopolitan 

 communities are the result. 



It is obvious that the development on the land is the reciprocal of 

 that in the sea. When the seas are extended and their life is tending 

 toward cosmopolitanism, the lands are dissevered, and their life is tend- 

 ing toward provincialism, and vice versa. When, however, the land is 

 greatly extended, it is usually accentuated by mountain ranges, and 



