THE ULTERIOR BASIS OF TIME DIVISIONS 457 
earth’s history, are therefore made upon the surface of the 
peripheral terrace and of its inland extension, and hence this 
becomes preéminently a critical geologic zone. 
To follow out the sequence of a typical cycle, let it be sup- 
posed that a circumcontinental submarine platform of ample 
dimensions has been developed, and that it is peopled bya 
fauna comparable to its extent and resources. It has been sug- 
gested that a typical crust movement has for its major feature 
the depression of the sea bottom and an increase in the capacity 
of the basin. Let such a movement succeed. The effect of this, 
whether it involves one ocean basin or a]ll, must be the with- 
drawal into itself of water from the submerged platforms of all 
the continents alike, since the oceans are connected. If the 
basin movement has sufficient magnitude to draw down the sea 
surface to the terrace edge, the shallow water zone becomes 
narrowed toa mere strip on the rapidly shelving abysmal face 
of the terrace, as illustrated in Fig. 3. 
Fic. 3.—a@ Former sea level. & Succeeding sea level. c Former ample shallow- 
water tract. d@ Succeeding constricted shallow-water tract. 
The ample fauna of the previous broader tract is thus forced 
into the constricted zone and brought under the direst stress of 
competition and scant room. The destruction of the larger part 
is inevitable, and the residue is forced to undergo repressive evo- 
lution to meet the severe conditions of the new environment. 
As this is common to all continents, it constitutes a comprehen- 
sive evolution of the severely competitive phase. There would, 
to be sure, be such exceptions as the local variations from the 
typical configuration of the continental border afford. These 
might be very considerable. Portions of the continents may have 
been previously carried down to moderate depths in the sinking 
of the ocean basins and may become shallow water ground by 
