780 EDMUND W. SINNOTT 



distribution or fossil remains, of a considerable degree of 

 antiquity. 



To corroborate this testimony as to the relative rapidity of 

 evolution in herbs and in woody plants, data as to their actual 

 rate of change today would be highly desirable; but this is very 

 difficult to obtain. As far as differences in "variability," using 

 the term in its broadest sense, are concerned, the two growth 

 forms seem nearly equal. In both there are many highly variable 

 types and many of great constancy. In the floras of three repre- 

 sentative regions — Eastern North America, Australia, and Ceylon — 

 the proportion of varieties and named forms among the woody 

 species is found to be practically the same as among herbs. Nor 

 is there a radical difference between the two in the extent of cross- 

 pollination by insects, although in temperate regions this is some- 

 what more common among herbs than among trees and shrubs. 

 The difference in length of generation to which we have called 

 attention is probably the most important factor in determining 

 the rate at which they have evolved. 



To whatever cause we may attribute it, however, there seems 

 to be little doubt that during the evolution of the angiosperms 

 the primitive, woody element has been developed very much more 

 slowly than the more recent, herbaceous one; and it is this differ- 

 ence which gives us a hint as to the antiquity of the whole group. 

 We find in the angiosperm flora today (dicotyledons alone con- 

 sidered) over 4,200 genera of trees or shrubs, as opposed to only 

 2,600 genera of herbs. We may be reasonably sure that practically 

 all of these 2,600 genera of herbs have been developed since the 

 beginning of the Tertiary; and if we assume that herbs are pro- 

 ducing new types only twice as fast as trees and shrubs — surely a 

 conservative estimate — we must believe that only about 1,300 

 woody genera have been evolved during the same time. The 

 evolution of the 4,200 genera of woody plants at present existing, 

 to say nothing of the great numbers which have been lost through 

 extinction (by which trees and shrubs have suffered much more 

 than herbs), would therefore require a period at least thrice the 

 length of the Tertiary. If the common assumption that the 

 Tertiary was approximately as long as the Cretaceous is correct, 



