ON THE HYPOTHESIS OF EVOLUTION". 143 



seems equally improbable when we consider that the characters in 

 which the parent's advance has appeared are rarely of a nature to 

 increase those facilities. 



The nearest approach to an explanation that can be offered 

 appears to be somewhat in the following direction : 



There is every reason to believe that the character of the 

 atmosphere has gradually changed during geologic time, and that 

 various constituents of the mixture have been successively re- 

 moved from it, and been stored in the solid material of the 

 earth's crust in a state of combination. Geological chemistry has 

 shown that the cooling of the earth has been accompanied by the 

 j)recipitation of many substances only gaseous at high tempera- 

 tures. Hydrochloric and sulphuric acids have been transferred 

 to mineral deposits or aqueous solutions. The removal of carbonic- 

 acid gas and the vapor of water has been a process of much slower 

 progress, and after the expiration of all the ages a proportion of 

 both yet remains. Evidence of the abundance of the former in 

 the earliest periods is seen in the vast deposits of limestone rock ; 

 later, in the prodigious quantities of shells which have been 

 elaborated from the same in solution. Proof of its abundance 

 in the atmosphere in later periods is seen in the extensive de- 

 posits of coal of the Carboniferous, Triassic, and Jurassic periods. 

 If the most luxuriant vegetation of the present day takes but fifty 

 tons of carbon from the atmosphere in a century, per acre, thus 

 producing a layer over that extent of less than a third of an inch 

 in thickness, what amount of carbon must be abstracted in order 

 to produce strata of thirty-five feet in depth ? No doubt it occu- 

 pied a long period, but the atmosphere, thus deprived of a large 

 proportion of carbonic acid, would in subsequent periods undoubt- 

 edly possess an improved capacity for the support of animal life. 



The successively higher degree of oxidization of the blood in 

 the organs designed for that function, whether performing it in 

 water or air, would certainly accelerate the performances of all 

 the vital functions, and among others that of growth. Thus it 

 may be that acceleration can be accounted for, and the process of 

 the development of the orders and sundry lesser groups of the 

 Vertebrate kingdom indicated ; for, as already pointed out, the 

 definitions of such are radically placed in the different structures 

 of the organs which aerate the blood and distribute it to its 

 various destinations. 



But the great question. What determined the direction of this 



