388 BIRDS. 



birds the hallux is wholly wanting, or is rudimentary. In the 

 Emeu, Cassowary, Bustards, and other genera, the hallux is 

 invariably absent, and the foot is three-toed. In the Ostrich 

 both the hallux and the next toe (" index ") are wanting, and 

 the foot consists simply of two toes, these being the outer toe 

 and the one next to it. 



As regards the geological distribution of Birds, there are 

 many reasons why we should be cautious in reasoning upon 

 merely negative evidence, and more than ordinarily careful not 

 to infer the non-existence of birds during any particular geolo- 

 gical epoch, simply because we can find no positive evidence 

 for their presence. As Sir Charles Lyell has well remarked, 

 " the powers of flight possessed by most birds would insure 

 them against perishing by numerous casualties to which quad- 

 rupeds are exposed during floods ; " and, " if they chance to 

 be drowned, or to die when swimming on water, it will scarcely 

 ever happen that they will be submerged so as to become pre- 

 served in sedimentary deposits," since, from the lightness of the 

 bones, the carcase would remain long afloat, and would be 

 liable to be devoured by predaceous animals. As, with a few 

 utterly trivial exceptions, all the deposits in which fossils are 

 found have been laid down in water, and more especially as 

 they are for the most part marine, these considerations put for- 

 ward by Sir Charles Lyell afford obvious ground against the 

 anticipation that the remains of birds should be either of fre- 

 quent occurrence or of a perfect character in any of the fossil- 

 iferous rocks. In accordance with these considerations, as a 

 matter of fact, most of the known remains of birds are either 

 fragmentary, 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 (fig. 331) impressed upon 

 certain sandstones in the valley of the Connecticut River in 

 the United States. These sandstones are almost certainly 

 Triassic, and if the ornithic character of these footprints be 

 admitted, then Birds date their existence from the commence- 

 ment 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, 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 facie presumption 

 that the authors of these prints were birds. 



