THE SUCCESSION OF ANIMAL FORMS 183 



and how strongly does it show the independence in some sense 

 of merely physical agencies on the part of the manifestations 

 of life ! 



We have naturally been occupied hitherto with the lower 

 tribes of animals and with plant life, because these are pre- 

 dominant in the early ages of the earth. Let us turn now to 

 the history of vertebrate or back-boned animals, which presents 

 some peculiarities special to itself. Many years ago Pander l 

 described and figured from the Cambro-silurian of Russia, a 

 number of minute teeth, some conical and some comb-like, 

 which he referred to fishes, and to that low form of the fish 

 type represented by the modern lampreys. Much doubt was 

 thrown on this determination, more especially as the teeth 

 seemed to be composed not of bone earth, but of carbonate of 

 lime, and it was suggested that they may have belonged to 

 marine worms, or to the lingual ribbons of Gastropod mol- 

 lusks. Some confirmatory evidence seems to have been sup- 

 plied by the discovery of great numbers of similar forms in the 

 shales of the coal formation of Ohio, by the late Dr. Newberry. 

 I have had an opportunity to examine these, and find that they 

 consist of calcium phosphate, 2 or bone earth, and that their 

 microscopic structure is not dissimilar from that of the teeth 

 of some of the smaller sharks (Diplodus) found with them. I 

 have therefore been inclined to believe that there may have 

 already been, even in the Cambrian or Lower Silurian seas, 

 true fishes, related partly to the lampreys and partly to sharks ; 

 so that the history of the back-boned animals may have gone 

 nearly as far back as that of their humbler relations. This 

 conjecture has recently received further support from the 

 discovery in rocks of Lower Silurian age, in Colorada of a 

 veritable bone bed, rich in fragmentary remains of fishes. 



1 More recently Rohan has described conical teeth (St. Petersburg 

 Academy, 1889), but I have not seen his paper. 



2 Analysis of Dr. B. J. Harrington. 



