Chap. XIV. 
RECAPITULATION. 
465 
proved; and when they do spread, if discovered in a 
geological formation, they will appear as if suddenly 
created there, and will be simply classed as new species. 
Most formations have been intermittent in their ac¬ 
cumulation ; and their duration, I am inclined to believe, 
has been shorter than the average duration of specific 
forms. Successive formations are separated from each 
other by enormous blank intervals of time; for fossili- 
ferous formations, thick enough to resist future de¬ 
gradation, can be accumulated only where much sedi¬ 
ment is deposited on the subsiding bed of the sea. 
During the alternate periods of elevation and of station¬ 
ary level the record will be blank. During these latter 
periods there will probably be more variability in the 
forms of life; during periods of subsidence, more ex¬ 
tinction. 
With respect to the absence of fossiliferous formations 
beneath the lowest Silurian strata, I can only recur to 
the hypothesis given in the ninth chapter. That the 
geological record is imperfect all will admit; but that 
it is imperfect to the degree which I require, few will 
be inclined to admit. If we look to long enough in¬ 
tervals of time, geology plainly declares that all species 
have changed; and they have changed in the manner 
which my theory requires, for they have changed slowly 
and in a graduated manner. We clearly see this in 
the fossil remains from consecutive formations invariably 
being much more closely related to each other, than are 
the fossils from formations distant from each other in 
time. 
Such is the sum of the several chief objections and 
difficulties which may justly be urged against my theory; 
and I have now briefly recapitulated the answers and 
explanations which can be given to them. I have felt 
these difficulties far too heavily during many years to 
X 3 
