136 RELATIVE GEOLOGICAL ANTIQUITY OF TREES. 



the air purer and fitted for the respiration of a higher order 

 of animals. At length appeared the first inhabitant of the 

 land a reptile! It is well known to Naturalists that in 

 reptiles and fishes the venous and arterial blood become mixed 

 together, the former being only partially returned to the lungs. 

 The respiration in these creatures is therefore imperfect, or 

 the blood is only partially oxygenated. Now, as the condition 

 of the atmosphere was such as to furnish life only to reptiles, 

 which do not require much oxygen, the presumption is, that 

 carbonic acid was still the predominating element. Man would 

 have been stifled at once in such a world. And this state of 

 things appears to have continued throughout the whole of 

 that vast interval of time immediately succeeding the deposi- 

 tion of the Palaeozoic rocks, which is called by Geologists the 

 SECONDARY FORMATION, and which Agassiz has very appro- 

 priately termed the " Age of Keptiles." This period includes 

 all the rock-formations from the New Eed Sandstone to the 

 Chalk. 



During all this time, the land was not continental, but the 

 earth continued to exhibit the appearance of an archipelago 

 of greater and smaller islands, which were generally without 

 mountains, presenting a low, flat surface. Naturalists are 

 forced to these conclusions by the consideration of the fossil 

 remains of plants and animals contained in the Secondary 

 rocks. This swampy condition of things undoubtedly pre- 

 vailed throughout the early part of the Secondary era ; but 

 afterward elevated table-lands, mountains, and hills diversified 

 the features of the landscape, the land continually encroaching 

 on the territories of the watery element, which, nevertheless, 

 still occupied the greater portion of the earth's surface. 



Hence it is that the predominant forms are amphibious 

 reptiles of monstrous size, which probably took their maxi- 

 mum development during the Oolitic period, creatures fitted 

 to inhabit both land and water. The birds, too, appear to 

 have been gigantic, and to have belonged to the class of 

 waders, which at present tenant low, swampy grounds on the 

 margin of rivers and bays. This is proved by their long 

 bones and immense strides. Professor Hitchcock discovered 



