128 EVOLUTION, GENETICS, AND EUGENICS 



ment, and sediment always arranges itself in layers or strata. In 

 sedimentary or stratified rocks fossils are found. The commonest 

 rocks of this sort are limestone, sandstone, and shales. Limestone is 

 formed chiefly of carbonate of lime; sandstone is cemented sand, and 

 shales, or slaty rocks, are formed chiefly of clay. 



"The formation of sedimentary rocks has been going on since land 

 first rose from the level of the sea; for water has always been wearing 

 away rock and carrying it as sediment into rivers, and rivers have 

 always been carrying the worn-off lime and sand and clay downward 

 to lakes and oceans, at the bottoms of which the particles have been 

 piled up in layers and have formed new rock strata. But geologists 

 have shown that in the course of the earth's history there have been 

 great changes in the position and extent of land and sea. Sea bottoms 

 have been folded or upheaved to form dry land, while regions once 

 land have sunk and been covered by lakes and seas. Again, through 

 great foldings in the cooling crust of the earth, which resulted in 

 depression at one point and elevation at another, land has become 

 ocean and ocean land. And in the almost unimaginable period of 

 time which has passed since the earth first shrank from its hypo- 

 thetical condition of nebulous vapor to be a ball of land covered with 

 water, such changes have occurred over and over again. They have, 

 however, mostly taken place slowly and gradually. The principal 

 seat of great change is in the regions of mountain chains, which, in 

 most cases, are simply the remains of old folds or wrinkles in the 

 crust of the earth. 



'•'When an aquatic animal dies, it sinks to the bottom of the lake 

 or ocean, unless, of course, its flesh is eaten by some other animal. 

 Even then its hard parts will probably find their way to the bottom. 

 There the remains will soon be covered by the always dropping sedi- 

 ment. They are on the way to become fossils. Some land animals 

 also might, after death, get carried by a river to the lake or ocean, 

 and find their way to the bottom, where they, too, will become fossils, 

 or they may die on the banks of the lake or ocean and their bodies 

 may get buried in the soft mud of the shores. Or, again, they are 

 often trodden in the mire about salt springs or submerged in quick- 

 sand. It is obvious that aquatic animals are far more likely to be 

 preserved as fossils than land animals. This inference is strikingly 

 proved by fossil remains. Of all the thousands and thousands of 

 kinds of extinct insects, mostly land animals, comparatively few speci- 

 mens are known as fossils. On the other hand, the shell-bearing 



