BIRDS. 251 



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 third and fourth digits. 



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 

 geological 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 quadrupeds 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 sub- 

 merged so as to become preserved in sedimentary deposits," 

 since, from the lightness of the bones, the carcass would 

 remain long afloat, and would be liable to be devoured by 

 predaceous animals. As, with a few utterly trivial excep- 

 tions, 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 forward by Sir 

 Charles Lyell afford obvious ground against the anticipation 

 that the remains of birds should be either of frequent 

 occurrence or of a perfect character in any of the fos- 

 siliferous 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. 585) impressed 

 upon certain sandstones in the vaUey of the Connecticut 

 river in the United States. These sandstones are almost cer- 

 tainly Triassic ; and if the ornithic character of these foot- 

 prints 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 Palseozoic 

 epoch. 



