THE ANCESTRY OF THE VERTEBRATE^ 91 



tropical and temperate countries. On certain parts 

 of the coast of Northern Canada the cliffs are formed 

 of rocks belonging to a stratum lying at the base 

 of the Cambrian and the very oldest in which fossils 

 of any description are to be found. In the cliffs it 

 is possible to chip out very perfect fossil shells of 

 the Lingula which lived on this coast in the pre- 

 Cambrian period; from the sand at the foot of the 

 cliffs we can dig out living specimens of Lingula, and 

 if we should examine the fossil shells and those of 

 the living animal with the minutest care we would 

 not be able to detect the smallest difference, down 

 to the finest striae, between them. We have here 

 revealed to us a fact which should never be lost sight 

 of in considering the evolution of animal life, a fact 

 which proves how short a period of the whole time in 

 which life has existed on the planet is represented 

 by the fossil-bearing rocks, and how stable and per- 

 sistent certain types of animals have remained in 

 the midst of the sweeping changes proceeding around 

 them. 



