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beneath the waters of a single ocean, and that an ocean richly 
tenanted by living beings, during the whole of the vast lapse of 
time between the commencement of the Upper Silurian and 
the close of the Devonian period ; whilst there is no proof of 
any considerable pauses in the process of sedimentation during 
the same period. Here, therefore, if anywhere, we ought to 
find proofs of evolution, if such a process really has taken 
place; and I shall immediately proceed to examine shortly 
some of the evidence that we have on this head. In the mean- 
while it may be noticed that there is another respect in which 
the Upper Silurian and Devonian rocks of North America are 
peculiarly fitted to throw light upon this inquiry. Mr. Darwin 
has pointed out that richly fossiliferous deposits have been 
formed mainly during periods in which the sea-bottom was 
undergoing subsidence ; but he remarks that during subsi- 
dence few new varieties or species will be formed, owing to the 
deepening of the sea and the consequent decrease in the inha- 
bited area and the number of inhabitants. The bearing of this 
remark upon Mr. Darwin’s views is obvious ; since sediments 
accumulated during subsidence, when few new species arc 
formed, could not be expected to yield many, or any, inter- 
mediate forms. Under any circumstances, I should not attach 
as much weight to the latter half of the above observation as 
Mr. Darwin seems inclined to do; but, at any rate, it does not 
apply to the case 1 have chosen. There is good evidence that 
the Upper Silurian and Devonian rocks of North America 
were laid down in an area of almost continued subsidence; but 
there is also good ground for believing that the accumulation of 
sediment kept pace, approximately, with the rate of subsidence; 
so that the depth of the sea remained tolerably constant, and 
there was no marked decrease in the size of the inhabited area 
and the number of inhabitants. We have also evidence that 
during the greater part of this period the sea was sufficiently 
shallow to admit of the existence of a profuse and varied 
marine fauna; and there is ample proof of the continual intro- 
duction of new species and varieties. 
5. The Absence of Numerous Intermediate Varieties in any 
Single Formation . — It is freely conceded that one of the greatest 
difficulties which Mr. Darwin’s theory has to overcome, is 
found in the fact that we do not find in the limits of any single 
formation “ closely graduated varieties between the allied 
species which lived at its commencement and at its close.” The 
essence of this difficulty lies in the words “ closely-graduated”; 
for we do find in any single formation certain intermediate 
forms, which may perhaps support a partial theory of evolution, 
