IH LECTURES ON EVOLUTION 61 



agencies, either out of the waste and washing of 

 the dry land, or else by the accumulation of the 

 exuviae of plants and animals. Many of these 

 strata are full of such exuviae the so-called 

 " fossils." Remains of thousands of species of 

 animals and plants, as perfectly recognisable as 

 those of existing forms of life which you meet 

 with in museums, or as the shells which you pick 

 up upon the sea-beach, have been imbedded in 

 the ancient sands, or muds, or limestones, just as 

 they are being imbedded now, in sandy, or clayey, 

 or calcareous subaqueous deposits. They furnish 

 us with a record, the general nature of which can- 

 not be misinterpreted, of the kinds of things that 

 have lived upon the surface of the earth during 

 the time that is registered by this great thickness 

 of stratified rocks. But even a superficial study of 

 these fossils shows us that the animals and plants 

 which live at the present time have had only a tem- 

 porary duration ; for the remains of such modern 

 forms of life are met with, for the most part, only 

 in the uppermost or latest tertiaries, and their 

 number rapidly diminishes in the lower deposits of 

 that epoch. In the older tertiaries, the places of 

 existing animals and plants are taken by other 

 forms, as numerous and diversified as those which 

 live now in the same localities, but more or less 

 different from them ; in the mesozoic rocks, these 

 are replaced by others yet more divergent from 

 modern types; and, in the paleozoic formations, the 



