GENERAL INTRODUCTION 13 



each family respectively, for increasing the area of its spore-bearing 

 surface, are sharply confined to such families. The special features 

 indicated are distinctly marked only in the most typical species, 

 but every family includes species liovering on the border-line 

 between one family and another, and it is often a purely personal 

 opinion as to whether a given species should be included in one 

 family or another. As an illustration, the genus Dcedalea (Plate 

 XXXI, Fig. 5) hovers between Polyporaceas and Agaricacea;. The 

 hymenium is intermediate in structure between true pore and gill 

 arrangement, in some specimens pores predominate, in others the 

 arrangement is mostly gill-like. On account of its woody con- 

 sistency and general durability, Dcedalea is placed in the Poly- 

 poracese. There are no such things as sharply defined families, or 

 genera, or species in nature ; sucli exist only in books. 



