XI PHENOMENA OF ORGANIC NATURE 377 



some particular bed or stratum which should con- 

 tain the remains of those creatures with which 

 organic life began upon the earth. And if we did 

 so, and if such forms of organic life were pre- 

 servable, we should have what I would call his- 

 torical evidence of the mode in which organic life 

 began upon this planet. Many persons will tell 

 you, and indeed you will find it stated in many 

 works on geology, that this has been done, and 

 that we really possess such a record ; there are 

 some who imagine that the earliest forms of life 

 of which we have as yet discovered any record, are 

 in truth the forms in which animal life began upon 

 the globe. The grounds on which they base that 

 supposition are these : That if you go through 

 the enormous thickness of the earth's crust and 

 get down to the older rocks, the higher vertebrate 

 animals the quadrupeds, birds, and fishes cease 

 to be found ; beneath them you find only the in- 

 vertebrate animals ; and in the deepest and lowest 

 rocks those remains become scantier and scantier, 

 not in any very gradual progression, however, 

 until, at length, in what are supposed to be the 

 oldest rocks, the animal remains which are found 

 are almost always confined to four forms Oldhatnia, 

 whose precise nature is not known, whether plant 

 or animal ; Lingula, a kind of mollusc ; Trilobites, 

 a crustacean animal, having the same essential 

 plan of construction, though differing in many 

 details from a lobster or crab ; and Hymenocaris, 



