HI LECTURES ON EVOLUTION 93 



at the present day, the investigations of such 

 anatomists as Rutimeyer and Gaudry have tended 

 to fill up, more and more, the gaps in our existing 

 series of mammals, and to connect groups former- 

 ly thought to be distinct. 



But I think it may have an especial interest if, 

 instead of dealing with these exam pies, which would 

 require a great deal of tedious osteological detail, 

 I take the case of birds and reptiles ; groups which, 

 at the present day, are so clearly distinguished from 

 one another that there are perhaps no classes of 

 animals which, in popular apprehension, are more 

 completely separated. Existing birds, as you are 

 aware, are covered with feathers ; their anterior 

 extremities, specially and peculiarly modified, are 

 converted into wings, by the aid of which most of 

 them are able to fly ; they walk upright upon two 

 legs ; and these limbs, when they are considered 

 anatomically, present a great number of exceeding- 

 ly remarkable peculiarities, to which I may have 

 occasion to advert incidentally as I go on, and 

 which are not met with, even approximately, in 

 any existing forms of reptiles. On the other hand, 

 existing reptiles have no feathers. They may have 

 naked skins, or be covered with horny scales, or 

 bony plates, or with both. They possess no wings ; 

 they neither fly by means of their fore-limbs, nor 

 habitually walk upright upon their hind-limbs ; 

 and the bones of their legs present no such modifi- 

 cations as we find in birds. It is impossible to 



