EVIDENCE OF FOSSIL REIVLVINS 81 



he cannot explore vast areas of the present ocean beds 

 which were formerly dry land and the homes of now- 

 extinct animals. Thus the field of investigation is 

 seriously restricted at the outset, but the naturahst 

 finds his work still more limited, in so far as much of the 

 dry land itself is not accessible. The perennial snows 

 of the Arctic region render it impossible to make a 

 thorough search in the frigid zone, and there are many 

 portions of the temperate and torrid zones that are 

 equally unapproachable for other reasons. "Rut even 

 where exploration is possible, the surface rocks are the 

 only ones from which remains can be readily obtained, 

 for the layers formed in earher ages are buried so deeply 

 that their contents must remain forever unknown in 

 their entirety. Only a few scratches upon the earth's 

 hard crust have been made here and there, so it is small 

 wonder that the complete series of extinct organisms 

 has not been produced by the palaeontologist. 



A brief survey of the varied groui)s of animals them- 

 selves is sufficient to bring to hght many biological 

 reasons w^hich account for still more of the vacant 

 spaces in the palaeontological record. We would hardly 

 expect to find remains of ancient microscopic animals 

 like the protozoa, unless they possessed shells or other 

 skeletal structures which in their aggregate might form 

 masses like the chalk beds of Europe. Jellyfish and 

 w^orms and naked mollusks are examjiles of the numerous 

 orders of lower animals having no hard parts to be 

 preserved, and so all or nearly all of the extinct sj^ecies 

 belonging to these groups can never be known. But 

 when an animal like a clam dies its shell can resist the 

 disintegrating effects of bacteria and other organic 



G 



