INTRODUCTION 7 



a certain degree of uniformity over a large part of the 

 earth's surface. It is essentially a swamp-flora, and we 

 know little or nothing of the vegetation of the uplands. 

 If we obtained all our knowledge of the existing flora 

 from plants growing in such localities as the Great 

 Dismal Swamp near the coast of the Southern United 

 States, or from the mangrove-vegetation' of tropical 

 shores, our ideas would evidently be one-sided. On 

 the other hand, when it is argued that on the hills of 

 the Coal-period there may have been flowers and trees 

 and shrubs of the higher Angiospermous families, that, 

 surely, is a wild imagination. The flora of swamps, 

 however peculiar, is made up of plants belonging to 

 the same main groups as those of other habitats, and 

 in so far as main groups alone are in question, we 

 may take the Coal-flora, such as it has come down to 

 us, as fairly representative of its period. 



Before beginning our detailed work, we must say 

 something as to the modes of preservation of fossil 

 plants, without some knowledge of which we can form 

 no idea of the relative value of the evidence before us. 



There are two different modes of preservation, which 

 we must keep perfectly distinct in our minds. These 

 have been termed incrustation and petrifaction. In- 

 crusted remains include almost all the specimens of 

 fossil plants familiar to the general public. In the 

 case of incrustation, the parts of plants were merely 

 incased in the mineral substance, and any of the larger 

 cavities they may have contained filled with it. Their 

 organic tissue became converted into structureless coal, 

 if it did not previously decay altogether. Hence, 

 incrusted specimens give us impressions or casts, 



