92 LIFE : OUTLINES OF GENERAL BIOLOGY 



most adult Amphibians, most Reptiles, many Birds, and most 

 Mammals. 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 imjx)rtant to bear in mind that many a stock may, in the 

 course of its evolution, have passed through a variety of environ- 

 ments. 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 freshwater 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 firma. 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 

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

 lingered by the way, many have diverged into culs-de-sac, 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 when littoral 

 animals forsook the shore for the land, via freshwater 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 thusojxened the way to a higher and more vigorous life. 



Ri-TUKNiNG TO THE Sea. — Pliny divided animals into Aquatilia, 

 Terrestria, and Volatilia, the creatures of water, earth, and air. 

 But among the "Aquatilia" it is necessary to distinguish the prim- 

 arily and secondarily aquatic. Thus no one supposes that jelly- 

 fishes and starfishes and cuttlefishes, to take only three examples, 

 ever lived anywhere but in water; whereas whales, turtles, and sea- 

 snakes are typfs whose ancestors lived on land. The primarily 

 aquatic animals breathe the oxygen that is dissolved in the water, 

 and they are cold-blooded, their body-temperature tending to be 

 the same as that of the surrounding medium. Another feature in the 

 primarily marine backboneless animals, and in the gristly fishes like 

 sharks and skates, is the osmotic equilibrium between the fluids of 



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