REPTILES AND AMPHIBIANS 123 



the earliest animal remains. Here we find a yawning 

 gap. No one can say what has become of the missing 

 strata. Many think that they are at the bottom of the 

 sea ; others see them in the upper layers of the non- 

 fossiliferous rocks, which have destroyed all the remains 

 of animals in the incalculable lapse of time, and owing 

 to pressure and decomposition. However that may be, 

 we must take into account the fact that the whole period 

 from the first appearance of life on the earth up to 

 the development of these highly organised animals is 

 hidden from us. 



Moreover, our discoveries in the periods known to 

 us are very defective. 



In the fossil remains or impressions of former animals 

 we have, as a rule, only the hard parts of organisms. 

 Hence animals that had no hard parts could leave no 

 trace of their existence. There is also another 

 circumstance that tells us the remains we have represent 

 only a small portion of the animals that lived at that 

 time. The strata that contain the fossils are formed 

 exclusively in water. The rivers carry stones with 

 them from the mountains, grind them up into fine mud, 

 and convey this to the sea, where it sinks to the bottom 

 and covers up any remains of animals that lie there. 

 These deposits of mud are gradually converted into solid 

 rock containing the animal remains ; and when the sea 

 recedes, the rock becomes the basis of new land, and 

 may be elevated into a mountain by the creasing of the 

 earth's surface. If one of these strata is left dry for a 

 long period, and then covered by the sea once more, a 

 new stratum is formed on top of it. But there will be 



