Chap. X.] GEOUPS 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 Palaeontology, 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 

 require 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 

 discovered 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 preservation 

 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 ? Not a frag- 

 ment 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 durino- the 

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