272 ON THE ORIGIN OF SPECIES 



forms, which would be able to spread rapidly and widely 

 throughout the world. 



I will now give a few examples to illustrate these 

 remarks, and to show how liable we are to error in 

 supposing that whole groups of species have suddenly 

 been produced. I may recall the well-known fact that 

 in geological treatises, published not many years ago, 

 the great class of mammals was always spoken of as 

 having abruptly come in at the commencement of the 

 tertiary series. And now one of the richest known 

 accumulations of fossil mammals, for its thickness, 

 belongs to the middle of the secondary series ; and 

 one true mammal has been discovered in the new red 

 sandstone at nearly the commencement of this great 

 series. Cuvier used to urge that no monkey occurred 

 in any tertiary stratum ; but now extinct species have 

 been discovered in India, South America, and in 

 Europe even as far back as the eocene stage. Had 

 it not been for the rare accident of the preservation 

 of footsteps in the new red sandstone of the United 

 States, who would have ventured to suppose that, 

 besides reptiles, no less than at least thirty kinds of 

 birds, some of gigantic size, existed during that period? 

 Not a fragment of bone has been discovered in these 

 beds. Notwithstanding that the number of joints 

 shown in the fossil impressions correspond with the 

 number in the several toes of living birds' feet, some 

 authors doubt whether the animals which left the 

 impressions were really birds. Until quite recently 

 these authors might have maintained, and some have 

 maintained, that the whole class of birds came sud- 

 denly into existence during an early tertiary period ; 

 but now we know, on the authority of Professor Owen 

 (as may be seen in Ly ell's Manual), that a bird 

 certainly lived during the deposition of the upper 

 greensand. 



I may give another instance, which from having 

 passed under my own eyes has much struck me. In 

 a memoir on Fossil Sessile Cirripedes, I have stated 

 that, from the number of existing and extinct tertiary 



