318 SKETCHES OF CREATION. 



scending in regular order, as in a succession of dominant 

 ideas, each in its own age expressing itself in more than 

 one organic type. Thus, in the reign of reptiles, the rep- 

 tilian idea was dominant, and we find it invading the 

 structure of the contemporaneous fishes. Afterward the 

 avian or ornithic idea became dominant, and reptiles were 

 endowed with wings, and even with feathers — if we may- 

 credit the reptilian character of the Archseopteryx of Solen- 

 hofen. Still later, the mammalian idea became dominant, 

 and the forms of the Ichthyosaurus and JPlesiosaurus, but 

 more especially of the terrestrial Peinosaurs, indexed the 

 impress of that idea upon the reptilian class. Even in the 

 Age of Molluscs, the dominant idea was expressed in the 

 bivalve nature of the Ostracoid Crustaceans. 



The forms styled " synthetic" or " comprehensive" types 

 may perhaps be generalized under the formula of dominant 

 ideas. Comprehensive types are those in which certain 

 characteristics of a group are ingrafted upon a distinct 

 though kindred stock. The Ganoid fishes are of this kind, 

 since they combine reptilian with fish-like features. The 

 Labyrinthodonts were comprehensive types, because they 

 were Amphibians with the scaly covering of Reptiles. The 

 Lepidodendra of the Coal era combined the characteristics 

 of the Cryptogams with the foliage and general habits of 

 the Conifers. Such a synthesis of types seems to be occa- 

 sioned by the overlapping of consecutive ideas in time — a 

 penumbra occurring while the last dominant idea is passing 

 under the shadow of the coining one. The Pterosaurs, or 

 flying reptiles, were the most marvelous of all comprehen- 

 sive or penumbral types. On the basis structure of a rep- 

 tile we find ingrafted the head and neck of a bird, the 

 trunk and tail of a quadruped, and the leathery wings of a 

 bat ; while, not improbably, their feet were furnished with 

 a web ; so that these creatures Avere fitted for all elements, 



