XV THE PFIILOSC^PHY OF ZOOLOGY 647 



would leave out of account the extreme incniupleteness of tiic 

 record of the history of life on the globe whicli 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 hanl 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 preservation 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 tiood 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 

 aoencies 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 faunjB (and flora?) of the various geological periods. 

 It is manifest, from Avhat 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 



