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 Ave 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 faunae 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 30 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 



