THE SUCCESSION OF ANIMAL FORMS ' 1 77 



numerous race modifications. My own provisional conclusion, 

 based on the study of Palaeozoic plants, is that the general law 

 will be found to be the existence of distinct specific types, in- 

 dependent of each other, but liable in geological time to a 

 great many modifications, which have often been regarded as 

 distinct species.^ 



While this unity of successive faunae at first sight presents 

 an appearance of hereditary succession, it loses much of this 

 character when w^e consider the number of new types introduced 

 without apparent predecessors, the necessity that there should 

 be similarity of type in successive faunae on any hypothesis of 

 a continuous plan ; and above all, the fact that the recurrence 

 of representative species or races in large proportion marks 

 times of decadence rather than of expansion in the types to 

 which they belong. To turn to another period, this is very 

 manifest in that singular resemblance which obtains between 

 the modern mammals of South America and Australia, and 

 their immediate fossil predecessors — the phenomenon being 

 here manifestly that of decadence of large and abundant 

 species into a few depauperated representatives. This will be 

 found to be a very general law, elevation being accompanied 

 by the apparent abrupt appearance of new types and decadence 

 by the apparent continuation of old species, or modifications 

 of them. 



This resemblance with difference in successive faunas also 

 connects itself very directly with the successive elevations and 

 depressions of our continental plateaus in geological time. 

 Every great Palaeozoic limestone, for example, indicates a 

 depression with succeeding elevation. On each elevation 

 marine animals wxre driven back into the ocean, and on each 

 depression swarmed in over the land, reinforced by new 

 species, either then introduced, or derived by migration from 

 other localities. In like manner, on every depression, land 

 * '* Geological History of Plants." 



