606 IBS (On MCV a CAMHS EIRIEION. 
fore develop its fauna in measurable independence. Each 
embayment will become the generating area of a provincial 
fauna. If nowa period of quiescence ensues and systematic 
continental evolution proceeds, these embayments will become 
extended landward and grow into extensive epicontinental gulfs 
and perhaps at length into broad epicontinental seas, and their 
faunas will expand accordingly. In the progress of this devel- 
opment they may come into conjunction with each other and a 
commingling and conflict of faunas ensue, resulting in the evolu- 
tion of anew assemblage of life of a composite type. 
Whether the internal progression reaches this stage or not, 
the development of the sea-shelves must at length attain astage 
such that coastal migration will become free and the faunas of 
the embayments become commingled by coastwise extension. 
The ideal result of this line of progression is the evolution at 
length of a general fauna of the expansional type and the con- 
current elimination or fusion of the provincial features, for the 
line of progress is essentially expansional, and the result is 
expansional evolution. It differs only from an expansional evo- 
lution starting from a general restrictional evolution in the com- 
mingling and conflict of well-differentiated faunas resulting from 
provincial development. 
At the close of the Silurian period the sea appears to have 
been drawn away from the land into the critical attitude here 
indicated, and the basin of the St. Lawrence Gulf and probably 
that of Hudson’s Bay and perhaps other embayments on the 
borders of the continent, appear to have furnished refuges for 
the retiring fauna of the Silurian period, and to have become 
areas in which the origination of provincial faunas took place. 
The consecutive series of sediments of the St. Lawrence embay- 
ment, though not yet perfectly investigated, give good grounds 
for the belief that the transition of the Silurian fauna into the 
Helderberg fauna took place there. After its provincial char- 
acter had been fully assumed and the re-advance of the sea 
opened the way into the interior through the Champlain tract, it 
reinvaded the interior basin and left its record as a distinctive 
