DISTRIBUTION. 321 



has everywhere resulted from the complete melting-up ot 

 beds of detritus, originally deposited by water. How long the 

 reactions of the Earth's molten nucleus on its cooled crust, 

 have been thus destroying the records of Life which this cooled 

 crust entombed, it is impossible to say ; but there are strong 

 reasons for believing that the records which remain, bear but 

 a small ratio to the records which have been destroyed. Thus 

 we have but extremely-imperfect data for any conclusions 

 respecting the distribution of organic forms in Time. Some 

 few generalizations, however, may be regarded as established. 



One is, that the plants and animals now existing, mostly 

 differ from the plants and animals wdiich have existed. 

 Though there are species common to our present Fauna and 

 to past Faunas ; yet the fades of our present Fauna differs, 

 more or less, from the fades of each past Fauna. On carry- 

 ing out the comparison, we find that past Faunas differ from 

 each other ; and that the differences between them are pro- 

 portionate to their degrees of remoteness from each other in 

 Time, as measured by their relative positions in the sediment- 

 ary series. So that if we take the assemblage of organic 

 forms living now, and compare it with the successive assem- 

 blages of organic forms that have lived in successive geologic 

 epochs, we find that the farther we go back into the past, the 

 greater does the unlikeness become : the number of species 

 and genera common to the compared assemblages, becomes 

 smaller and smaller ; and the assemblages differ more and 

 more in their general characters. Though a species of 

 brachiopod now extant, is almost identical with a species 

 found in Silurian strata, and though between the Silurian 

 Fauna and our own, there are sundry common genera of mol- 

 luscs ; it is still undeniable that there is a proportion between 

 lapse of time and divergence of organic forms. 



This divergence is comparatively slow and continuous, 

 where there is continuity in the geological formations ; but is 

 sudden and comparatively wide, wherever there occurs a 

 great break in the succession of strata. The contrasts which 



