DISTRIBUTION OF AVES IN TIME. 



533 



have been laid down in water, and more especially as they are 

 for the most part marine, these considerations put forward by 

 Sir Charles Lyell afford obvious ground against the anticipa- 

 tion that the remains of birds should be either of frequent 

 occurrence or of a perfect character in any of the fossiliferous 

 rocks. In accordance with these considerations, as a matter 

 of fact, most of the known remains of birds are either frag- 

 mentary or belong to forms which were organised to live a 

 terrestrial life, and were not adapted for flight. 



The earliest remains which have been generally referred to 

 birds are in the form of footprints impressed upon certain 

 sandstones in the valley of the Connecticut River in the 

 United States. These sandstones are almost certainly Triassic ;' 



Fig. 233. Footprint supposed to belong to a Bird. Triassic Sandstones of 

 Connecticut. 



and if the ornithic character of these footprints be admitted, 

 then Birds date their existence from the commencement of the 

 Mesozoic period, and, for anything we know to the contrary, 

 may have existed during the Palaeozoic epoch. 



The evidence as to the ornithic character of the footprints 

 in the American Trias is as follows : 



Firstly r , The tracks are, beyond all question, those of a biped 

 that is to say, of an animal which walked upon two legs. 

 No living animals walk habitually upon two legs except Man 

 and Birds, and therefore there is a prima fade presumption 

 that the authors of these prints were birds. 



Secondly, The impressions are mostly tridactylous that is to 



