VASCULAR FLOWERLESS PtANTS. 91 



on the stem of tree-ferns, and these cicatrices are very con- 

 spicuous in the fossil fern-stems, and form a valuable and ac- 

 curate indication of their true character. 



134. Certain conditions necessarily produce certain phe- 

 nomena. This philosophical axiom is especially applicable to 

 the phenomena of vegetation. Certain plants require certain 

 conditions of temperature, air, light, and moisture, in order to 

 insure their development from the spore or seed. But these 

 fossil arborescent ferns and Equisetacese, although analogous 

 in their growth to the existing arborescent species in tropical 

 countries, are altogether specifically different. They have had 

 their final development, and are now extinct. The earth, once 

 their home, is now their sepulchre. Hence it is evident that 

 the conditions under which they were developed exist no 

 longer; and when, along with these extinct plants, we find the 

 remains of animals equally low in the scale of organization, and 

 which have no living representatives in the zoology of any 

 country, we have indubitable evidence of a system of organic 

 change, which has been probably contemporaneous with the 

 changes which have undoubtedly taken place in inorganic 

 nature. 



135. The history of the earth has been written by the 

 Author of nature in characters the most striking and impres- 

 sive, in its strata, which have been justly termed the "leaves 

 of the stone book." But these characters can only be inter- 

 preted by a careful and accurate study of the living creation, 

 and the existing laws which govern inorganic matter. Not 

 only every world in space, but every atom in the universe, 

 moves according to fixed and unchanging laws, and the inte- 



