Chap. X]. GROUPS OF ALLIED SPECIES. 79 



sea like the logger-headed duck, and ultimately to rise 

 from its surface and glide through the air? 



I will now give a few examples to illustrate the 

 foregoing remarks, and to show how liable we are to 

 error in supposing that whole groups of species have 

 suddenly been produced. Even in so short an interval 

 as that between the first and second editions of Pictet's 

 great work on Paleontology, published in 1844-46 and 

 in 1853-57, the conclusions on the first appearance and 

 disappearance of several groups of animals have been 

 considerably modified; and a third edition would re- 

 quire still further changes. I may recall the well- 

 known fact that in geological treatises, published not 

 many years ago, mammals were always spoken of as 

 having abruptly come in at the commencement of the 

 tertiary series. And now one of the richest known ac- 

 cumulations of fossil mammals belongs to the middle of 

 the secondary series; and true mammals have been dis- 

 covered in the new red sandstone at nearly the com- 

 mencement 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, as far back as the miocene 

 stage. Had it not been for the rare accident of the pres- 

 ervation of footsteps in the new red sandstone of the 

 United States, who would have ventured to suppose that 

 no less than at least thirty different bird-like animals, 

 some of gigantic size, existed during that period? Kot 

 a fragment of bone has been discovered in these beds. 

 Not long ago, palaeontologists maintained that the 

 whole class of birds came suddenly into existence during 

 the eocene period; but now we know, on the authority 

 of Professor Owen, that a bird certainly lived during 



