] INTRODUCTION 9 



one hundred and sixteen species, thus representing a total of 

 three hundred and forty-eight sections, and that the diagnoses 

 were at first of a purely tentative character demanding frequent 

 revision and extension, it will be understood that the enormous 

 mass of detail involved not only presented considerable diffi- 

 culties but demanded the expenditure of much time and most 

 patient and painstaking effort. The concurrent prosecution of 

 paleontological studies, in which a very critical means of differ- 

 entiation was called for, because of structural defects arising 

 from decay and other influences attendant upon the process of 

 fossilization, fortunately gave just the insight into the require- 

 ments of critical diagnoses which was required. It soon became 

 clear that certain anatomical features stood forth with very great 

 prominence, and that they could be successfully employed for 

 the recognition of primary and secondary divisions of the group ; 

 while other less prominent characters naturally fell into the 

 categories of those which define genera and species. Upon this 

 basis it was soon possible to differentiate the various genera with 

 accuracy, as already set forth in previous publications (44). The 

 systematic treatment of the genera of the North American Coni- 

 ferales then elaborated has been in constant and successful use 

 for several years, as applied to the determination of both fossil 

 and recent woods. Such experience has shown the classification 

 to be substantially correct with respect to the accuracy of the 

 diagnoses and the efficiency of the artificial key connected there- 

 with. A few minor changes have been found necessary, and 

 these have been introduced in connection with the more recent 

 revisions. Later experience, especially as derived from a more 

 critical study of the anatomical details, has shown the need of a 

 revision of the generic and specific sequences, as embodied in 

 recently published papers (59) and now incorporated here. As 

 it now stands, comparatively slight familiarity with the classifi- 

 cation will enable one to refer most woods to their appropriate 

 genera without hesitation. Thus Taxus, Torreya, and Pseudo- 

 tsuga may be isolated at once by the single character which they 

 possess in common, tracheids with spirals, - - while the last 



