Ill 



THE RISE OF LAND LIFE 



EARLIEST Palaeozoic time seems to have been charac- 

 terized by seas of wide area. Its deposits are largely 

 limestones laid down in fairly deep water. The earli- 

 est known foot print of a terrestrial vertebrate is in sediment 

 belonging to the upper devonian period; the animal had al- 

 ready emerged on land, perhaps during the lower devonian 

 period, when the land began to rise, and shallow water, or 

 bays and rivers were extensive. 



The weight of geological evidence seems to favor the view 

 that both sharks and ganoids developed in fresh water out 

 of some more primitive immigrant ancestor from the sea.^ 

 Here there is still much uncertainty. But we may be fairly 

 confident that the ganoid stock at least was chiefly represented 

 in the rivers of the rising continents in early devonian times. 

 The ganoide, sturgeon, garpike, and others, show wide variety 

 of form and structure. They differed greatly in the form 

 and thickness of their scaly defensive armor. I believe that 

 we may safely imagine that the well defended forms would 

 hold the channels of the rivers and the lagoons, while those 

 with least defensive armor would be driven into the shallower 

 waters and swamps. 



The climate of this period seems to have been semi-arid, 

 seasons of pouring rain and floods alternating with drought. 

 Under such circumstances the air-bladder capable of inhal- 

 ing atmospheric air would have been of great and steadily in- 

 creasing use and value, especially in the headwater marshes, 

 where in the hot summers the gases of decaying vegetation 



1 H. 462. 



31 



