Reflection* on Primitive Vegetation* 7 



can be absorbed by a plant. Now, even a small proportion 

 of carbonic acid in the atmosphere, is generally unfavourable 

 to animal life, and especially of the more perfect animals, as 

 the mammifers and birds ; this proportion, on the contrary, is 

 highly favourable to the growth of vegetables, and if we admit 

 that there existed a greater quantity of this gas in the primi- 

 tive atmosphere of the globe, than there is in our atmosphere, 

 we may consider it as one of the principal causes of the lux- 

 uriant vegetation of those distant periods. 



This mass of vegetables, which, from their simplicity and 

 uniformity would have been so little fitted to furnish materi- 

 als for the sustenance of animals, differing in their structure, 

 like those which now exist, may have prepared such a state of 

 things as was necessary for a more varied creation, by purify- 

 ing the air from the excess of carbonic acid it then contained ; 

 and if we would indulge the feeling of pride which sometimes 

 leads man to think that all nature has been created for his 

 benefit, we may suppose that this first vegetable creation, pre- 

 ceding, by so many ages, the appearance of man upon the 

 earth, may have been intended to bring about that state of the 

 atmosphere, which would be necessary for his existence, and 

 the accumulation of those immense beds of fuel, which his 

 industry was afterwards to avail itself of. 



But independently of this difference in the constitution of 

 the atmosphere, which the formation of these immense depo- 

 sits of fossil coal, renders extremely probable, may not the 

 nature of vegetables themselves, furnish us with some data 

 concerning the other physical conditions, to which the sur- 

 face of the earth was subjected during this period ? 



What is now going on in different parts of the globe, may 

 throw some light on this question. 



The study of the geographical distribution of plants belong- 

 ing to the same tribes that composed the entire vegetation of 

 the coal period, may, in effect, point out to us the state of cli- 

 mate, and, consequently, the physical causes, which favour 

 the increase of size and number in these vegetables ; and we 

 may, in all probability, conclude from it, that the same causes 

 occasioned their preponderance at the epoch under review. 



We see, for example, that the Filices, Equisetacece, and 

 Eycopodiacece, attain a greater height in proportion as they 

 approach the equatorial regions. So that it is only in the 

 hottest parts of the globe we can find the arborescent ferns, 

 which, uniting with the upright and majestic form of the palm 

 tree, the elegant foliage of the common fern, we have already 

 mentioned as existing in the coal strata. In the same regions, 

 the Equiseta and Lycopodia attain to a height, double and 



b 4 



