HI LECTURES ON EVOLUTION 79 



died before the falls had cut their way back 

 through the gorge of Niagara ; and, indeed, it has 

 been determined that, when these animals lived, the 

 falls of Niagara must have been at least six miles 

 further down the river than they are at present. 

 Many computations have been made of the rate 

 at which the falls are thus cutting their way back. 

 Those computations have varied greatly, but I 

 believe I am speaking within the bounds of 

 prudence, if I assume that the falls of Niagara 

 have not retreated at a greater pace than about 

 a foot a year. Six miles, speaking roughly, are 

 30,000 feet ; 30,000 feet, at a foot a year, gives 

 30,000 years; and thus we are fairly justified in 

 concluding that no less a period than this has 

 passed since the shell-fish, whose remains are left 

 in the beds to which I have referred, were living 

 creatures. 



But there is still stronger evidence of the long 

 duration of certain types. I have already stated 

 that, as we work our way through the great series 

 of the Tertiary formations, we find many species 

 of animals identical with those which live at the 

 present day, diminishing in numbers, it is true, 

 but still existing, in a certain proportion, in the 

 oldest of the Tertiary rocks. Furthermore, when 

 we examine the rocks of the Cretaceous epoch, 

 we find the remains of some animals which the 

 closest scrutiny cannot show to be, in any im- 

 portant respect, different from those which live at 



