4 
AMERICAN GEOLOGY. 
expires. It has retired more inward toward the earth’s core, 
and it is girt about by stronger and stronger bands; but it yet 
preserves its outward vents, and often warns us of its power in 
the trembling of its bands and the molten rocks which flow 
from the lire chasms which it opens. 
The interpretation of the varied phenomena to which we 
have just alluded, must be in accordance with human experi¬ 
ence and observation. Observations turned to those phenomena 
which belong to each of the periods in the earth’s history, 
prove that the plan of creation—the ideas which that plan 
expresses—is a unity. It proves more than this, that the pres¬ 
ent is only a part of the past, and belongs to it. As the whole of 
a thing is made up of its parts, and is imperfect in the absence 
of one, so the present is imperfect without the past, and the 
past would be imperfect without the present. The interpreta¬ 
tion of the present is perfected only by reference to the past, 
and the past would be unintelligible without the present. Each 
period then is a fragment; but the present is a greater fragment 
than all the past put together. The present does not date its 
beginning with man, but with the earliest species of plants and 
animals which now live, and whose primordial forms are not 
as yet extinct. 
§ 6. The life of plants and animals is controlled by a single 
element; that is oxygen. The adaptation of organs is in 
accordance with its properties. It has always been so. We 
presume, too, that its supply has been constant—that there has 
been no period when its quantity was either greater or less 
than it is now. A different view is not sustained by the fact 
that at one period huge lizards predominated in its fauna, for 
analogy proves that this class would have perished with a less 
proportion than that which exists in our atmosphere at the 
present time. 
In cases of this kind reasoning from special structural affinities 
to general physical conditions is not always safe. The lias and 
oolitic periods abound in the remains of cold-blooded animals, 
whose respiratory apparatus was undoubtedly imperfect, like the 
