

REVIEW OF THE HISTORY OF LIFE. 255 



a gradual change in the fossils by the drojiping out and intro- 

 duction of species one by one. Thus in the whole of the 

 great TaJjcozoic period, both in its fauna and flora, we have 

 a continuity and similarity of a most marked character. 



Tiiere is, indeed, nothing to i)reclude the supposition that 

 many forms reci^oned as species are really only race modifica- 

 tions. My own provisional conclusion, based on the study of 

 Palcxozoic plants, published many years ago,i is that the general 

 law will be fount! to be the existence of distinct specific types 

 independent of each other, but liable in geological time to 

 minor modifications, which have often been regarded as distinct 

 s])ecies. 



While this unity of successive faunai at first sight presents 

 an appearance of hereditary succession, it loses much of this 

 character when we consider the number of new types in- 

 troduced without apparent predecessors, or ceasing witliout 

 successors, and the almost changeless persistence of other 

 ty])es; the necessity that there should be similarity of type in 

 successive fauna) on any hypothesis of a continuous plan ; 

 and, above all, the fact that the recurrence of ^-cpresentative 

 sjiecies or races in large proportion marks times of decadence 

 rather than of expansion in the types to which they belong. To 

 turn to a later period, this is very manifest in that singular 

 resemblance which obtains between the modern mammals of 

 South America and Australia and their immediate fossil pre- 

 decessors—the phenomenon being here manifestly that of 

 decadence of large and abundant species into a few dei)au- 

 perated representatives. This will be found to be a very 

 general law, elevation being accompanied by the abrupt 

 appearance of new types, rnd decadence by the apparent 

 continuation of old species, or modifications of them. 



"''his resemblance with differenre in successive fimnrc also 

 connects itself very directly vvi.h the successive elevations and 

 depressions of our continental plateaus in geological time. 

 ^ Report on Devonian Plants of Canada, 187 1. 



