THE AERIAL FAUNA. 759 



which lives in moist earth, some of the Planarians, Nema- 

 todes, Leeches, Chaetopods, and other "worms," a few 

 Crustaceans like the wood lice (Oniscus), many insects and 

 Arachnids, a legion of slugs and snails, most adult 

 Amphibians, most Reptiles, many Birds, and most Mam- 

 mals. Among Vertebrates certain fishes are of interest in 

 having learned to gulp mouthfuls of air at the surface of 

 the water, to clamber on the roots of the mangrove trees, 

 or to lie dormant through seasons of drought. But among 

 Vertebrates, Amphibians were the first successfully to 

 make the transition from water to dry land. 



It is important to bear in mind that many a stock may, in the 

 course of its evolution, have passed through a variety of environments. 

 Thus the thoroughly aquatic Cetaceans were probably derived from a 

 land stock common to them and to the Ungulates, and may have passed 

 through a fresh water stage. Without going further back, we have here 

 an illustration of the zigzag course of evolution. 



We cannot believe in any abrupt transition from the shore to terra 

 fir ma. It has been a slow ascent, slow as the origin of dry land 

 itself. Thus, mud-inhabiting worms, dwellers in damp humus, bank- 

 frequenting animals, those which find a safe retreat in rottenness or 

 within bolder forms, dot the path from the shore inland. Many have 

 lingered by the way, many have diverged into cul-de-sacs, many have 

 been content to keep within hearing of the sea's lullaby, which soothed 

 them in their cradles. 



Simroth, in his work on the origin of land animals, seeks to show 

 that hard skins, cross-striped muscle, brains worthy of the name, red 

 blood, and so on, were acquired as the transition to terrestrial life 

 was effected. Let us take the last point by way of illustration. Iron 

 in some form seems essential to the making of haemoglobin, but iron 

 compounds are relatively scarce and not readily available in the sea, 

 they are more abundant in fresh water, and yet more so as the land is 

 reached. Therefore it is suggested that it was as littoral animals 

 forsook the shore for the land, via fresh water paths, that iron, in some 

 form, entered into their composition, became part and parcel of them, 

 helped to form haemoglobin or some analogous pigment, and thus opened 

 the way to a higher and more vigorous life. 



The Aerial Fauna. 



The last region to be conquered was the air. Insects 

 were the first to possess it, but it was long before they were 

 followed. The flying fishes flapped their fore-fins above 

 the foam as they leapt ; the web-footed tree frogs, Draco 

 volans, with skin spread out on elongated ribs, and various 



