SOURCE OF EVOLUTION OF PROVINCIAL FA UNAS 605 
in a much more limited number but better adapted to the new 
conditions, will have been introduced. Such restrictive condi- 
tions appear to have been prevalent ina pronounced degree at 
the close of the Paleozoic era and less notably at the close of 
the Ordovician period and at other times. But in neither of 
these cases were the repressional conditions complete, and it is 
improbable that the ideal conditions of repression here sketched 
were ever fully realized. 
(3) Conditions favorable to the evolution of provincial faunas. — 
It is obvious that if the sea shore be drawn far down the abysmal 
slope of the deal sea-shelf, moderate warpings Olgene .cOn- 
tinental platform will have little or no effect upon the con- 
ditions of faunal development, for whether the shore stands 
high or low upon this abysmal face the shallow-water tract will 
remain a mere ribbon. But if, on the other hand, the sea be 
withdrawn merely to the angle of the sea-shelf the relations 
between sea and land will be critical and every warping of the 
surface platform will be decisive either in emphasizing the 
restrictional influence or in relieving it. To illustrate by a 
specific case: suppose the sea level to lie accurately at the 
angle of the ideal sea-shelf, and that portions of the con- 
tinental platform are warped upward to the amount of 500 feet, 
while alternating portions are warped downward to an equal 
Peounts) ihe shore lime mathe former case will lie along the 
abysmal face and the shallow-water tract will be narrow. In the 
latter case the shore line will be thrown out upon the upper 
surface of the sea-shelf and the shallow-water tract will be 
relatively wide. If the sea-shelf be ideal its upper surface will 
have a very gentle slope and the downward warping of 500 feet 
would carry the shore line well inland and give a notable embay- 
ment favorable to the perpetration and development of shallow- 
water life. Under such conditions of alternate warping up and 
down the continental platform would be bordered by embay- 
ments favorable to life, separated by narrow shore tracts which 
would be largely prohibitive of free migration of shallow-water 
life between the embayments. Each embayment will there- 
