59 2 MANUAL OF ZOOLOGY. 



abundance of food in any particular locality. Other birds, 

 however, at certain seasons of the year undertake long jour- 

 neys, usually uniting for this purpose into large flocks. These 

 birds such as the swallows, for instance are properly called 

 " migratory birds " (aves migratorice). The movements of 

 these birds are conditioned by the necessity of having a cer- 

 tain mean temperature, and consequently they leave the cold 

 regions at the approach of winter, and return again for the 

 warmer season. 



DISTRIBUTION OF BIRDS IN TIME. As regards the geolo- 

 gical distribution of Birds, there are many reasons why we 

 should be cautious in reasoning upon merely negative evi- 

 dence, 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 submerged so as to become pre- 

 served 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 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 adapted for flight. 



The earliest remains which have been generally referred to 

 birds are in the form of footprints (fig. 328) 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. 

 In the fact that these footprints are three-toed, and are cer- 

 tainly the tracks of bipedal animals, we have strong evidence 

 that they were produced by birds. On the other hand, it is 



