THE AMERICAN BOTANIST 127 



But whatever the cause of their origin, herbs have proved 

 themselves an exceedingly versatile and aggressive type of vege- 

 tation under almost all climatic conditions. The reasons for 

 this dominence are not far to seek. They are able not only to 

 thrive in cold and arid regions but, from the brevity of their 

 life cycle, can take advantage of temporarily favorable condi- 

 tions of any sort. Their evident and great superiority over 

 woody plants in rapidity of dispersal and ability to invade new 

 areas quickly is due in large measure to the fact that their in- 

 terval from seed to seed, instead of being many years, is only 

 a few months. Every seed may itself become a center of dis- 

 persal in a season's time. The amount of seed produced, too, 

 in proportion to the bulk of the plant body which has to be 

 developed is far greater among herbs than among woody 

 forms. Owing to the rapid multiplication of their generations, 

 herbs are capable of more rapid evolutionary change than are 

 trees or shrubs, and hence are able to adjust themselves more 

 rapidly to new conditions. With these various advantages, it 

 is not surprising that the herbaceous habit today characterizes 

 not only great numbers of the commonest and most dominant 

 native plant species in all parts of the world, but also that huge 

 array of hardy and ubiquitous plants which we know as weeds. 



This radical change in the growth habit of many plants 

 from the woody to an herbaceous type which has taken place 

 for the most part since the beginning of Tertiary time has not 

 failed to exert an important influence on animal life. It may 

 well be connected with the rapid evolution of mammals which 

 we know to have occurred after the early Tertiary. To quote 

 from Chamberlain and Salisbury : 



The earliest Eocene mammals were much more primitive 

 and obscurely differentiated that even those of the middle 

 Eocene, and this rapid backward convergence seems to point 

 to some set of conditions which caused an exceptionally rapid 

 development of the great class at this stage, whatever their 

 previous history had been. The coming into a new domain of 



