DISTRIBUTION. 405 



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 

 one another, and that the differences between them are pro- 

 portionate to their degrees of remoteness from one another 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 which 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, though between the Silurian Fauna 

 and our own there are sundry common genera of molluscs, 

 yet it is 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 

 thus arise, gradually or all at once, in formations that are 

 continuous or discontinuous, are of two kinds. Faunas of 

 different eras are distinguished partly by the absence from 

 the one of types present in the other, and partly by the 

 unlikenesses between the types common to both. Such con- 

 trasts between Faunas as are due to the appearance or disap- 

 pearance of types, are of secondary significance : they possibly, 

 or probably, do not imply anything more than migrations or 

 extinctions. The most significant contrasts are those between 

 successive groups of organisms of the same type. And 

 among such, as above said, the differences are, speaking 

 generally, small and continuous where a series of conformable 



