31 



dissolved away without leaving any traces of their existence. 

 The skeletons of birds are undoubtedly often absent under 

 circumstances that would lead us to infer the presence of birds, as 

 in the Eocene deposits of Monte Bolca, where feathers are 

 abundantly preserved. Again, in the Tertiary deposits of the 

 gypsum the footprints of birds occur in profusion, associated with 

 those of the multitudes of herbivorous and carnivorous animals 

 described by Cuvier. The bones of mammalian animals are pre- 

 served in the same strata with those of the reptiles that perished 

 with them in the overflowing waters charged with sulphuric acid, 

 the immediate cause of their destruction, and subsequently of the 

 formation of the gypseous marls. But not a single fragment of a 

 bird's skeleton has as yet been discovered after fifty years' search 

 in that particular locality. Eggs and feathers are found in the 

 marls of Ais, but no osseous fragments occur in the same strata. 

 The numerous footmarks imprinted on large areas of Triassic 

 sandstones in Connecticut, once regarded as those of wading or 

 wingless birds, but now referred to extinct bird-like reptiles, 

 afford another example of the existence of numbers of animals of 

 which at the time of their discovery we had no further record. 

 Thus, it is very evident that our knowledge is founded but on 

 shreds and patches, a link here and there, with many a gap 

 between, which if all the birds that once lived on the earth could 

 be restored to life might be bridged over. Then, the concep- 

 tion of the remote reptilian ancestry of all the varied existing 

 forms of diving, flying, perching, running, fruit eating, flesh 

 devouring, and insect feeding birds, with all their diversities of 

 colour, form, and habit would seem less marvellous and inexplic- 

 able. 



At least 12,000 species of existing birds are known, of 

 which perhaps two-thirds are enumerated in Mr. Bowdler Sharpe's 

 Catalogue of the Birds in the British Museum, in twenty-three 

 volumes. The fossil forms known are included in one volume by 

 Mr. Lydekker. 



The reptilian birds and bird-like reptiles of the Cretaceous 

 and Jurassic epochs illustrate a few of the intermediate steps in 

 the process of evolution, and were doubtless in turn the descen- 

 dants of that ancestral reptilian stock whence all the subsequent 

 birds and reptiles were successively evolved. The occurrence of 

 teeth in the immature parrot, webbed feet in the robin (first 

 noted by Agassiz), and the similarity of structure which at some 

 stages of growth is alike characteristic of the fore and hind limbs 

 of birds, afford further evidence of reptilian descent. Bat in 



