xv THE PHILOSOPHY OF ZOOLOGY 611 



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 others, subjected to the pressure 

 of enormous superincumbent masses, and perhaps acted upon by 

 intense heat and other agents of change, have become completely 

 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 to enable us to judge of the general character of 

 the faunae (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 

 successive 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 Mam- 

 mals. Birds are, however, very highly 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 as far back as the Permian. Amphibia, in the shape of 

 the Labyrinthodonts, first appeared in the Devonian ; while all the 

 earliest vertebrate remains in the Cambrian and Silurian forma- 

 tions appear to belong to the class of the Fishes. Within each of 

 these classes a progress is usually traceable from older, more 

 generalised types, along diverging lines, to the various specialised 

 forms existing at the present day. In some cases, however, notably 

 in the Amphibia, Reptilia and Aves, the orders first represented 

 have become entirely extinct, and have been succeeded by orders 

 that made their appearance on the scene at a comparatively late 

 period. 



R R 2 



