INTRODUCTIOX. 17 



time, or their ''geological distribution," belongs to the science of ' 

 ^^ Palcsontology " (Gr. palaios, ancient; niitn., beings; loffos, a dis- 

 course), and constitutes a subject so vast that nothing more can be 

 done here than to indicate one or two of the most elementary con- 

 siderations relating thereto. In the iirst place. Geology shows us 

 that a very large portion of the crust of the earth is composed of 

 rooks which existed originally in the form of sand, mud, clay, or 

 ooze, and which formed the floor of the ocean.. Other rocks can be 

 shown to have been originally formed by lakes or rivers, and some- 

 times we find what may be regarded as old land-surfaces or soils. 

 These various kinds of rock, in the second place, often contain in 

 their interior what are called " fossils " or " petrifactions," — in other 

 words, the remains or traces of animals and plants which lived at 

 the time when the rocks were in actual process of formation. In 

 rocks which have been formed in the sea, the fossils consist chiefly 

 of the skeletons of shell-fish, corals, sea-urchins, and other marine 

 animals ; in rocks which have been formed in lakes or rivers, we 

 have chiefly fresh-water shells and the skeletons of fresh-water 

 fishes ; and in ancient soils we find the remains of plants, along 

 with air-breathing animals, such as insects, spiders, or quadrupeds. 



We see, then, that animals existed upon the earth for a long 

 period before the appearance of such forms as are now familiar to 

 us ; and we have to note, in the third place, that not only are exist- 

 ing animals in many cases different from those which immediately 

 preceded them, but that the globe has really passed through a suc- 

 cession of periods, during each of which there flourished an assem- 

 blage of animals more or less peculiar to the period. In the later 

 periods of the earth's history, the animals which lived in the sea or 

 in lakes and rivers, or which roamed upon the land or disported 

 themselves in the air, present a geneial likeness to the animals now 

 in existence, though not identically the same. In the earlier 

 periods, again, the animals are not only " extinct," o;', in other 

 words, no longer existent, but they are very unlike any animals 

 which we see at the present day, and the older the period the 

 greater is this unlikeness. 



We have, finally, to remember, that though many extinct animals 

 are so peculiar that we have to place them in distinct families or 

 orders, there is at present no known fossil v hich camiot be referred 

 to one or other of the existing siih-kingdoms. We have therefore, 

 so far as our present knowledge goes, no proof of the former exist- 

 ence and disappearance of any " morphological type." 



