THE ANCESTRY OF BIRDS. 609 



and in our own time the tendency is very fully displayed among a 

 large number of forestine mammals. 



During the secondary ages, however, it was the reptiles which took to 

 thus developing a rudimentary flying-mechanism. Even at the present 

 day there are some modern lizards, the " flying-dragons " of popular natu- 

 ral history, which possess a parachute arrangement of the front ribs, and 

 are so enabled to jump lightly from branch to branch, somewhat in the 

 same manner as the flying-squirrels. But this is an independent and 

 comparatively late development of a flying apparatus among the rep- 

 tiles, quite distinct in character from those which were in vogue among 

 the real and much more terrible flying-dragons of the liassic and oolitic 

 age. Far the most remarkable of these predecessors of the true birds 

 were the pterodactyls whose bones we still find in our English cliffs at 

 Lyme Regis and Whitby ; creatures with a large reptilian head, fierce 

 jaws set with sharp-pointed teeth, and fore-arms prolonged into a great 

 projecting finger so as to support a membranous wing or fold of skin, 

 somewhat analogous to that of the bats. The pterodactyls do not 

 stand anywhere in the regular line of descent toward the true birds ; 

 but they are interesting as showing that a general tendency then ex- 

 isted among the higher reptiles toward the development of a flying 

 organ. In these frightful dragons, the organ of flight is formed by 

 an immense prolongation of the last finger on each fore-leg, to a length 

 about as great as that of the rest of the leg all put together. Between 

 this long bony finger and the hind-leg there stretched, in all probabil- 

 ity, a featherless wing like a bat's, by means of which the pterodactyl 

 darted through the air and pounced down upon its cowering victims. 

 As in birds, the bones were made very light, and filled with air instead 

 of marrow ; and all the other indications of the skeleton show that the 

 creatures were specially designed for the function of flight. Imagine 

 a cross between a vulture and a crocodile, and you have something 

 like a vague mental picture of a pterodactyl. 



But at the very time when the terrestrial reptilian type was 

 branching out in one direction toward the ancestors of the pterodac- 

 tyls, it was branching out in another direction toward the ancestors of 

 the true birds. In the curious lithographic slate of Solenhofen we 

 have preserved for us a great number of fossil forms with an extraor- 

 dinary degree of perfection ; and among these are several which help 

 us on greatly from the reptilian to the avian structure. The litho- 

 graphic slate is a member of the upper oolitic formation, and it is 

 worked, as its name implies, for the purpose of producing stones for 

 the process of lithography. But the same properties which make the 

 slate in its present condition take so readily the impress of a letter or 

 a sketch made it in its earlier condition take the impress of the vari- 

 ous organisms imbedded as they fell in its soft mud. Even the forms 

 and petals of early flowers washed down by floods into the half -formed 

 mud-bank" 1 , have been thus preserved for us with wonderful minute- 



YOL. XXIV. 39 



