NATURAL HISTORY OF CREATION. 



173 



defective children,* we can appreciate the important 

 effects of both these physical conditions in ordinary re- 

 production. Now there is nothing to forbid the sup- 

 position that the earth has been at different stages of 

 its career under different conditions, as to both air and 

 light. On the contrary, we have seen reason for sup- 

 posing that the proportion of carbonic acid gas (the 

 element fatal to animal life) w^as larger at the time of 

 the carboniferous formation than it afterwards became. 

 We have also seen that astronomei-s i-egard the zodiacal 

 light as a residuum of matter enveloping the sun, and 

 which was probably at one time denser than it is now. 

 Here we have the indications of causes for a progress in 

 the purification of the atmosphere and in the diffusion 

 of light during the earlier ages of the earth's history, 

 with which the progress of organic life may have been 

 conformable. An accession to the proportion of oxygen, 

 and the effulgence of the central luminary, may have 

 been the immediate prompting cause of all those advances 

 from species to species which Ave have seen, upon other 

 grounds, to be necessarily supposed as having taken place. 

 And causes of the like nature may well be supposed to 

 operate on other sphei'es of being, as well as on this. I 

 do not indeed present these ideas as furnishing the true 

 explanation of the progress of oi-ganic creation ; they are 

 merely thrown out as hints towards the formation of a 

 just hypothesis, the completion of which is only to be 

 looked for when some considerable advances shall have 

 been made in the amount and character of our stock of 

 knowledge. 



* Some poor people having taken up tlieir abode in the cells under 

 the fortifications of Lisle, tlie proportion of defective infants produced 

 by them became so great, that it was deemed necessary to issue an 

 order commanding these cells to be shut up. 



