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186 VEGETATION AND THE ATMOSPHERE. 
But the earth is quite old, and it is not impossible that its atmosphere has 
undergone, since the creation, progressive changes which have become very 
considerable through the lapse of so many ages. We have here a very curious ° 
question, which has been considered by M. Brougniart, and which we will 
proceed to study with him. The earth covers enormous, we might say inex- 
haustible, masses of carbon under the form of coal, anthracite, lignite and peat, 
and it cannot. be doubted for an ‘instant that these deposits are not the accumu- 
lated fossil remains of innumerable vegetables. Now there is for a plant but 
one single mode of acquiring carbon—to imbibe it in the form of carbonic acid 
from the air, and consequently all those masses of coal which cover Belgium, 
England, a large portion of America, and which are found at all points of 
the globe, were once diffused in a gaseous state through the atmosphere; 
they were there combined with oxygen, and the globe in the beginning was 
involved in an aeriform envelope which contained azote, a great deal of car- 
bonic acid, little or no oxygen. If we add that, at the moment, the earth was 
incandescent, we see that all the carbon must in effect at that temperature 
have been burned on contact with oxygen. 
Thus constituted, the earth cooled down; but the composition of its atmo- 
sphere rendered it uninhabitable for animals, since they had need of oxygen 
and there was none, since they would have been suffocated in the carbonic 
acid and azote which prevailed at the moment. Hence the first strata of sedi- 
mentary deposits contain no animals. In return, the earth was as favorably 
prepared for the production of plants as it was unfit for the nourishment of 
animals; it was soon, therefore, covered with luxuriant forests, whose remains, 
in accumulating, formed coal. We find therein all the species then living. 
There were gigantic eguisetums, arborescent ferns comparable to our oaks, and 
palms which towered above everything that the vegetable kingdom now offers 
us. And while these immense deposits were forming, oxygen, perpetually dis- 
engaged by the action of the sun, was gradually impregnating the atmosphere 
-and preparing it for the advent of the animal tribes. Of these, in due time, 
the first creations made their appearance, having since varied from age to age. 
At the epoch of the coal formations the forests were tenanted by huge reptiles, 
cold-blooded animals, for which little oxygen sufficed ; but it was not till after 
the nearly total disappearance of the carbonic acid that the earth witnessed 
the arrival of the mammifers, which had awaited a richer atmosphere. 
There are those at once timorous and ignorant who seriously ask what will 
become of the earth and themselves when mankind have burnt up all the coal. 
I will tell you, honest folks, what will become of us. The coal will have 
again been converted into carbonic acid, oxygen will have disappeared, and 
the creat vegetable tribes will return; but-if it is true, as they would persuade 
us, that the animal species, by growing gradually more perfect, have advanced 
from the primitive forms up to man, the return of the elements to their point 
of departure would bring man back to his origin by an inverted degenerescence. 
To have had crocodiles among our ancestors, be it so; but to see in perspec- 
tive a posterity composed of ichthyosauri, this certainly is the most disheart- 
ening of metempsychoses ! 
But to return to serious matters: if we are ignorant of the mechanism of 
the living organs, at least we know the functions they fulfil, and can express 
clearly the part which they play in the physical world. With the water and 
azotized substances which they take from the soil, with a gas which they col- 
lect in the air, vegetables compose organic matter, which they accumulate in 
their tissues and which they hold in reserve for the use of animals. The vege- 
table kingdom seems to be only a great laboratory, an atelier of production 
where every plant has the same function—that of forming materials as varied 
in their composition as are the forms of each one of them.. To this common 
