II PROGRESS OF PALEONTOLOGY 31 



reference to this particular case, that " bodies 

 which are altogether similar have been produced 

 in the same way." 1 Hence, since the glossopetrse 

 are altogether similar to sharks' teeth, they must 

 have been produced by sharklike fishes ; and 

 since many fossil shells correspond, down to the 

 minutest details of structure, with the shells of 

 existing marine or freshwater animals, they must 

 have been produced by similar animals ; and the 

 like reasoning is applied by Steno to the fossil 

 bones of vertebrated animals, whether aquatic or 

 terrestrial. To the obvious objection that many 

 fossils are not altogether similar to their living 

 analogues, differing in substance while agreeing in 

 form, or being mere hollows or impressions, the 

 surfaces of which are figured in the same way as 

 those of animal or vegetable organisms, Steno 

 replies by pointing out the changes which take 

 place in organic remains embedded in the earth, 

 and how their solid substance may be dissolved 

 away entirely, or replaced by mineral matter, 

 until nothing is left of the original but a cast, an 

 impression, or a mere trace of its contours. The 

 principles of investigation thus excellently stated 

 and illustrated by Steno in 1669, are those which 

 have, consciously or unconsciously, guided the 

 researches of palaeontologists ever since. Even 

 that feat of palaeontology which has so powerfully 



1 "Corpora sibi invicem omnino similia simili etiam modo 

 producta sunt " 



