6o8 THE POPULAR SCIENCE MONTHLY. 



may say that at an early period the central reptilian stock, consisting 

 of the long, lithe, four-legged forms like the lizards, still closely allied 

 in shape to their primitive newt-like and eel-like ancestors, began to 

 divide laterally into sundry important branches. Some of them lost 

 their limbs and became serpents ; others acquired bony body-cover- 

 ings and became turtles ; but the vast majority went off in one of two 

 directions, either as fish-like sea-saurians, or as bird-like land-saurians. 

 It is with this last division alone that Vv^e shall have largely to deal in 

 tracing out the pedigree of our existing birds. Their fossil remains 

 supply us with many connecting links which help us to bridge over 

 the distance between the modern representatives of the two classes. 

 It is true, none of these links can be said to occupy an exactly inter- 

 mediate place between reptiles and birds ; none of them can be re- 

 garded as forming an actual part of the ancestry of our own swallows 

 and pigeons : they are rather closely related collateral members of 

 the family than real factors in the central line of descent. But they 

 at least serve to show that, at and before the period when true birds 

 first appeared upon earth, many members of one great reptilian group 

 had made immense advances in several distinct directions toward the 

 perfected avian type. 



Clearly, the first step toward the development of a bird must con- 

 sist in acquiring a more or less upright habit : for the legs must be 

 well differentiated into a large hind pair and a free fore pair, before 

 the last can be further specialized into feathered wings ; and the body 

 must have acquired a forward poise before flying becomes a possible 

 mode of locomotion. Such an upright habit is first foreshadowed 

 in the larger-limbed and longer-legged lizards like the dinosaurians, 

 which walked to some extent erect, and more particularly in some 

 highly specialized reptiles like the iguanodon, which had large hind- 

 legs and small fore-legs, and could walk or hop on the hind-legs alone, 

 much after the fashion of a kangaroo, or still more of a jerboa or a 

 chinchilla. Now, it is noticeable that the tendency to acquire the 

 most rudimentary form of flying is common among animals of this 

 semi-erect habit, especially when they frequent forests and jump about 

 much from tree to tree. For example, among our modern mammals, 

 the squirrels are a race much given to sitting on their hind-legs and 

 using their paws as hands ; while they are also much accustomed to 

 jumping lightly from bough to bough : and some among them, the 

 flying squirrels, have developed a sort of parachute consisting of an 

 extensible skin between the fore and hind legs, which they use to 

 break their fall in descending to the ground. Again, among the lower 

 monkey-like animals, the so-called flying lemur or galeopithecus has 

 hit upon an exactly similar plan ; while, in the bats, a membrane which 

 may be fairly called a wing has been evolved to a very high degree of 

 perfection. Everywhere, the habit of living among trees or jumping 

 from rocks tends to produce either parachute or wing-like organs ; 



