58 



THE DARWINIAN THEORY 



of man, but almost to the first appearance of animal 

 life of which we have positive evidence. 



As examples of persistent types may be cited 

 Globigerina, which shares in the formation of chalk, 

 and is found in the Trias, or bottom of the Secondary 

 strata ; Limuhcs, the king-crab, which occurs in the 

 Trias, and is found on the American coast at the 



Fig. 4. 



Fig. 5. 



Dentalhitn. 



Lingula. 



present day ; Dentalium, the tusk-shell, an animal in 

 some respects between an oyster and a snail, with a 

 tubular conical shell about two inches long, by which 

 it burrows in the sand, is found in the Devonian and 

 perhaps Silurian strata ; the Pearly Nautilus, a rare 

 animal, is found in the lower Silurian and at the 

 present day ; Lingula, an animal with extreme 

 tenacity of life, is found in the lower Cambrian and 

 at the present day, and, so far as we can see, is un- 

 changed. (Figs. 4 and 5.) 



