xv THE PHILOSOPHY OF ZOOLOGY 627 



the history of life on the globe which is preserved to us in the 

 rocks. In the first place, there are many groups of animals and 

 plants which, owing to the absence of any hard supporting parts, 

 are incapable of leaving any recognisable trace of their former 

 existence in the form of fossils. Again, even in the case of such 

 as have such hard parts, the conditions necessary for their pre- 

 servation in deposits destined to be converted into rock cannot be 

 of very frequent occurrence ; and many forms might fail to be 

 preserved simply owing to the non-occurrence of such conditions. 

 In the case of land-animals, such as Mammals or Reptiles, for 

 example, when one of them dies, it is for the most part torn to 

 pieces, and even the bones destroyed by various carnivorous and 

 carrion- feeding creatures. Only now and again would it happen 

 that, by becoming buried in a morass, or swept away by a flood and 

 buried under alluvial deposits, such forms might be preserved. 



Again, great thicknesses of sedimentary strata, sometimes con- 

 taining fossils, can be shown to have become removed by the 

 agencies of denudation, or the various forces such as the action 

 of waves, tides, and currents in the sea, of rain and fresh-water 

 streams on the land -by which rock-masses are constantly, where 

 exposed, being worn away ; while other rocks, subjected to the 

 pressure of enormous superincumbent masses, and perhaps acted 

 upon by intense heat and other agents of change, have been com- 

 pletely metamorphosed their mineral constituents having become 

 re-arranged and what organic remains they may have contained 

 completely destroyed. Moreover, of the fossil-bearing rocks that 

 remain unaltered, only a small part can be said to have been 

 thoroughly explored for fossil-remains. 



Yet, notwithstanding these causes of imperfection in the record 

 of the succession of life on the earth preserved to us in the rocks, 

 there is sufficient evidence to enable us to judge of the general 

 character of the faunas (and florae) of the various geological periods. 

 It is manifest, from what has already been stated throughout the 

 earlier sections with regard to the geological history of each 

 phylum and class, that there has been a general progress in suc- 

 cessive eras from the simple to the more complex ; the higher 

 forms have, so far as the recorded facts enable us to judge, come 

 into existence later than the lower. The Vertebrata may be 

 taken as an example. There is no evidence of the existence of 

 the highest class the Mammalia earlier than the Triassic period 

 of the Mesozoic era. The case of the Birds appears at first sight 

 anomalous : Birds appear for the first time in deposits of Jurassic 

 age, and are therefore more recent than the oldest Mammals. 

 Birds are, however, very higly specialised Vertebrates, and should 

 it be proved that they appeared at a time when primitive Mammals 

 already existed, the separate evolution of the two classes from 

 lower forms would afford a sufficient explanation. Reptiles extend 



