480 THE DEPTHS OF THE SEA. [ CHAP. X. 
temperature in deep water to a like extreme degree. 
These abnormal temperatures are dependent upon 
the present distribution of sea and land; and I 
have already shown that we have evidence of many 
oscillations, in modern times geologically speaking, 
which must have produced totally different condi- 
tions of temperature over the same area. Accepting, 
as I believe we are now bound to do in some form, 
the gradual alteration of species through natural 
causes, we must be prepared to expect a total absence 
of forms identical with those found in the old chalk, 
belonging to groups in which there is sufficient 
structural differentiation to require or to admit of 
marked variation under altering circumstances. The 
utmost which can be expected is the persistence of 
some of the old generic types, and such a resemblance 
between the two faunze as to justify the opinion that, 
making due allowance for emigration, immigration, 
and extermination, the later fauna bears to the 
earlier the relation of descent with extreme modi- 
fication. 
I have already mentioned that one of the most 
remarkable differences between the recent Atlantic 
chalk-mud and the ancient white chalk is the total 
absence in the latter of free silica. It would seem, 
from the analysis of chalk, that silicious organisms 
were entirely wanting in the ancient cretaceous seas. 
In the chalk mud, on the other hand, silica is found 
in abundance, in most specimens to the amount of 
from 80 to 40 per cent. A considerable portion of 
this is inorganic silica—sand; and its presence is 
doubtless due to the circumstance that our dredgings 
have hitherto been carried on in the neighbourhood 
