314 PRINCIPLES OF ANIMAL BIOLOGY 



considerable proportion of fossil plant life consisting of molds and 

 casts. 



Subsequent Fate of Fossils. — Once a fossil is formed it is subject to 

 physical agencies that may destroy it completely or considerably distort 

 it. Formed under water, the rock strata in which fossils are now being 

 discovered were elevated and drained. In this process of elevation, 

 folding and breaking of the rocks, many fossils were injured or destroyed. 

 Circulating ground water may redissolve the material of which the fossils 

 are composed ; and if the material is replaced the internal structure of the 

 animal is likely to be lost. Erosion lays bare numerous fossils, which may 

 be weather-worn, or washed away; and if the fossils were in fragments the 

 parts are scattered beyond recovery. Pressure in the rock strata, due 

 in part to the weight of rock above, but perhaps more largely to shrinkage 

 of the sediment in drying-out, crushes many fossils, so that the latter 

 are often in fragments and far from representative of the original form of 

 the animals. 



Discovery of Fossils. — Fossils are not found everywhere, and it is 

 seldom that the paleontologist sets out to find them without some clue. 

 Natural erosion may lay bare deposits of great value, in which the fossils 

 are accidentally discovered by travelers or exploring parties. Excava- 

 tions for buildings, drainage canals and ditches, railroad cuts, mines and 

 quarries are prolific sources of first discoveries. Particularly valuable 

 leads are often followed up by scientific expeditions sent out by museums 

 and universities. Most of the valuable collections have been brought 

 together by this concentrated, directed effort, not by casual discoveries. 



Change in Species During Geological Time. — The primary fact of 

 paleontology, which makes the study of fossils both fascinating and im- 

 portant, is that the animals of the past were very often unlike the animals 

 of today. There are, it is true, many instances in which there is little 

 or no difference between extinct and living animals, especially in late 

 geological periods. Even in early periods some forms existed which were 

 very similar to animals of today. Lingula, one of the brachiopods still 

 living in marine waters, is almost identical with certain very ancient 

 fossil brachiopods. But among the older forms of life similarity to recent 

 forms is exceptional. 



Moreover, the living beings of one past age differed from those that 

 preceded or followed them. Examination of successively deeper and 

 deeper strata of rock reveals differences in the animals which lived when 

 those rocks were deposited. Animals which are abundant in the lower 

 (older) strata are wanting in those above, while new forms take their place 

 in the more recent rocks. So characteristic are these changes from one 

 stratum to another that geological time may be measured, or at least 

 divided, with reference to the fossils contained in the rocks. 



In order to be able to speak of the successive rock deposits geologists 



