PALEONTOLOGY 295 



inference is strikingly proved by fossil remains. Of all the 

 thousands and thousands of kinds of extinct insects, mostly 

 land animals, comparatively few specimens are known as fossils. 

 On the other hand, the shell-bearing mollusks and crustaceans 

 are represented in almost all rock deposits which contain any 

 kind of fossil remains. 



It is obvious that any portion of the earth's surface covered 

 by stratified rocks must have been at some time under water, 

 the bottom of a lake or ocean. If now this portion shows a 

 series of layers or strata of different kinds of sedimentary rocks, 

 it is evident that it must have been under water several times, 

 or at least under different conditions. It is also evident that 

 fossils found in this portion of the earth will contain remains 

 of only those animals which were living at the various times 

 this portion of the earth was under water. Of the animals 

 which lived on it when it was land there will be no trace, 

 except, possibly, a few land or fresh-water forms, which might 

 be swept into the sea or might be preserved in the mud of ponds. 



FIG. 176. Restoration of Dimorphodon macronyx. (After Seeley.) 



That is, instead of finding in the stratified rocks of any portion 

 of the earth remains of all the animals which have lived on that 

 portion since the earth began, we shall find, at best, only re- 

 mains of a few kinds of those animals which have lived on this 

 portion of the earth when it was covered by the ocean or by a 

 great lake. 



Thus, the great body of fossil remains of animals reveal only 

 a broken and incomplete history of the animal life of the past. 

 But the record ; so far as it goes, is an absolutely truthful one, 



