468 MANUAL OF ZOOLOGY. 



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 predacious 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 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 fossiliferous 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 organised for flight. 



The earliest remains which have been generally referred to 

 birds are in the form of footprints impressed upon certain sand- 

 stones 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 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, 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. 



Secondly, The impressions are mostly tridactylous that is 

 to say, formed by an animal with three toes on each foot, as is 

 the case in many Waders and most Cursorial birds. 



Thirdly, The impressions of the toes show the same numeri- 

 cal progression in the number of phalanges as exists in living 

 birds that is to say, the innermost of the anterior toes has 

 three phalanges, the middle one has four, and the outermost 

 toe has five phalanges. 



Taking this evidence collectively, it would have seemed, till 



