458 T. C. CHAMBERLIN 
the lowering of the sea level. But even then there would 
remain a community of dominant action that would give a 
decisive aspect to the progress of life and to the sedimentation 
on all continents alike. 
If now a long period of quiescence follows, a new universal 
terrace will begin to form and will extend its marine plain sea- 
ward and its baselevel inland until at length an ample zone for 
the evolution of a new shallow water fauna is provided. If to 
the cutting of the sea edge and the filling in of the sea basin 
there be added the settling of the continent, the sea may make 
a wide incursion upon the low parts of themland=asmit, cic 
Cretaceous times, and unusual facilities be thus afforded for that 
form of life-evolution which follows rich and genial conditions. 
Thus on the one hand the sinking sea bottom induces that 
form of evolution in which stress is the dominant factor, and on 
the other, quiescence induces that form of evolution in which 
new ground and rich opportunities constitute the dominant con- 
dition. Both of these follow simply and inevitably from the 
sinking of parts that have been already predominant in sinking, 
and from prolonged intervening stages of quiescence. Almost 
the only essential postulate of the one evolution isa periodic 
increase of sea basin capacity; of the other, periodic quiescence. 
No profound catastrophe is involved; rather on the contrary it 
is inhibited by the conditions postulated. 
Both the evolution of restrictive environment and the evolu- 
tion of expansive environment, in the opinion of the writer, are 
effective in the change of faunas, though their respective results 
may be as different as their modes. In the rhythmical action 
postulated there is an alternating application of these opposed 
evolutionary processes with the natural result of an effect of the 
maximum order; for the evolutionary effects of restrictive 
conditions are believed to reach their greatest magnitude when 
they follow conditions of expansion, and, reciprocally, expan- 
sive conditions realize their greatest results when they follow 
conditions of restriction. 
Such a succession of shallow sea incursions and withdrawals 
