196 



THE GEOLOGICAL HISTORY OF PLANTS. 



absolutely certain, since we know that at present one 

 genus may have considerable variety in its leaves, and, on 

 the other hand, that plants of different genera may be 

 very much alike in their foliage. There is, however, un- 

 doubtedly a likeness in plan or type of structure in leaves 

 of closely allied plants, and, therefore, if judiciously 

 studied, they can be determined with at least approxi- 

 mate certainty.* More especially we can attain to much 

 certainty when the fruits as well as the leaves are found, 

 and when we can obtain specimens of the wood, showing 

 its structure. Such corroboration is not wanting, though 

 unfortunately the leaves of trees are generally found 

 drifted away from the other organs once connected with 

 them. In my own experience, however, I have often 

 found determinations of the leaves of trees confirmed by 

 the discovery of their fruits or of the structure of their 

 stems. Thus, in the rich cretaceous plant-beds of the 

 Dunvegan series we have beech-nuts associated in the 

 same beds with leaves referred to Fagus. In the Laramie 

 beds I determined many years ago nuts of the Trapa 

 or water-chestnut, and subsequently Lesquereux found, 

 in beds in the United States, leaves which he referred to 

 the same genus. Later, I found in collections made on 

 the Eed Deer Kiver of Canada my fruits and Lesquereux's 

 leaves on the same slab. The presence of trees of the 

 genera Carya and Juglans in the same formation was in- 

 ferred from their leaves, and specimens have since been 

 obtained of silicified wood, with the microscopic structure 

 of the modern butternut. Still we are willing to admit 

 that determinations from leaves alone are liable to doubt. 



In the matter of names of fossil leaves, I sympathise 

 very strongly with Dr. Nathorst, of Stockholm, in his 



* Great allowance has to be made for the variability of leaves of the 

 same species. The modern hazel (C. rostrata) is a case in point. Its 

 leaves, from different parts of the same plant, are so dissimilar in form 

 and size that they might readily be regarded as of different species. 



