288 OUTLINES OF EVOLUTIONAKY BIOLOGY 



fortunate chance to find their way to some place where rock 

 formation of a suitable kind is going on. The remains of land 

 animals may be carried by rivers to some sea or lake and buried 

 in a suitable accumulation of mud or sand, but it is more likely 

 that they will not. Marine animals, provided they have hard 

 skeletons, have, of course, many more opportunities of attaining 

 a lasting monument, and we often find great thicknesses of rock, 

 such as chalk and limestone, composed almost entirely of their 

 remains. 



Even when the record has been successfully established, 

 however, it is liable to destruction by various agencies. The 



A. B. c. 



FIG. 134. Three Species of Trilobites. (From British Museum Guide.) 

 A., Agnostus princeps ; B., Olenus cataractes ; C., Staurocephalus murchisoni. 



rocks containing it may be uplifted above sea-level and planed 

 right away by sub-aerial denudation, or they may be sunk so 

 far beneath later accumulations as to be profoundly altered by 

 the action of heat and pressure, by which means any fossils 

 which they contain may be completely destroyed. Then, again, 

 we must remember that only a relatively small proportion of 

 the earth's crust is accessible for investigation. Deep-lying 

 rocks may be brought to the surface by upheaval, and denudation 

 of overlying strata, or they may be reached by deep mining, but 

 we know practically nothing of the strata which now lie beneath 

 the bed of the sea, and even the dry land has only been tentatively 

 scratched by inquisitive man in a few places. 



Even were the geological record far less perfect than it 

 actually is, therefore, its imperfection could, not legitimately be 



