INTRODUCTION. 



5 



and upon plants widely separated in a natural classification having the same form 

 of leaf. Intending students have been led to think that leaves may not be sepa- 

 rated into species when they are dissimilar in form, nor united in one when similar. 

 Surely, however, it does not follow that, because the task is difficult, nothing should 

 be attempted. Apart from their determination, if we regard leaves merely as signs, 

 and are indifferent, for instance, as to whether they belong to Oak, or Beech, or 

 Elm, or to common ancestors of these, they still possess much interest, since frequently 

 on their evidence alone the date of many a volcanic eruption, change of level, or silting 

 up of lakes, has to be fixed. Even in our own country, we see that the volcanic outbursts 

 of Mull and the North of Ireland, and the lake-system at Bovey-Tracey, have been 

 determined to be of Miocene age entirely on plant evidence. 



However great the difficulties may be in determining these fossil plants from the 

 isolated organs which alone for the most part we possess, the task is certainly not 

 altogether hopeless, Eortunately we are not wholly dependent upon leaves, but have 

 large series of fruits and seeds as well, and even occasional flowers to assist us. Were 

 we, therefore, to find leaves which, although seemingly of Oaks, for example, but which 

 might be leaves of other and widely separate families, we should hesitate how to 

 class them ; but if we find that acorns had been floated down by the same river 

 which brought the leaves, our doubts would be greatly removed. With the increasing 

 stock of knowledge such results may be hopefully looked for. But even where we have 

 nothing but detached leaves to deal with, much may be done. Many plants can be 

 recognised by the form of the leaves, still more by the venation, and their determination is 

 more certain when the texture is preserved. The latter is of great importance ; for 

 instance, the leaves of a species of Nettle and of a Cinnamon have the same venation and 

 form, yet owing to the difference in their texture they could, even if fossil, hardly be 

 mistaken. Texture, however, although indicated in the fossil, cannot always be reproduced 

 in the illustrations. Even the leaves of those plants which vary much can generally be 

 recognised, if a large series be examined, by their venation, though in outline they may 

 be quite dissimilar. The question is not, however, whether some plants so vary that it 

 is impossible to determine them from their leaves, but it would be important to determine 

 whether the species of the living genera to which these fossils have been referred are so 

 variable. The habit of collecting and attentively examining fossils from deposits of one 

 age, if extended over many years, induces so great a familiarity with their peculiarities of 

 texture and aspect that they become easily recognisable by minor difi'erences, Avhich 

 would escape even a botanical specialist who passed them under examination for the first 

 time. Again, considerable advantage is gained by attending to the general assemblage 

 of plants in a fossil flora. As an instance, we find at Bournemouth a leaf, hitherto 

 supposed to be that of a Castanea, associated abundantly and almost exclusively with 

 Palms and Ferns of tropical American type. The correctness of the determination 

 appears doubtful, since we have no precedent fo.v such a grouping, whereas a species 



