GROUPS OF ALLIED SPECIES. 335 



these birds their front limbs in this precise iatermediate 

 state of "neither true arms nor true wings?" Yet these 

 birds hold their place victoriously in the battle for life; for 

 they exist in infinite numbers and of many kinds. I do 

 not suppose that we here see the real transitional grades 

 through which the wings of birds have passed; but what 

 special difficulty is there in believing that it might profit 

 the modified descendants of the penguin, first to become 

 enabled to flap along the surface of the 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 fore- 

 going 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 

 Paleeontology, published in 184-t-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, pub- 

 lished 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 tlje 

 secondary series; and true mammals have 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, 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 fragment of bone has been dis- 

 covered 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 the deposition of the upper greensand; and 

 still more recently, that strange bird, the Archeopteryx, 



