48 CAUSES WHICH FAVOUR OR [pt. i 



But we must go on to consider the advantages or disadvan- 

 tages in the matter of spreading about the world that arise from 

 herbaceous or M-oody nature. It is clear that a herb will in general 

 go through its generations more rapidly than a shrub, and still 

 more quickly than a tree. A herb producing seed in its first 

 year may get three or more generations, and as many chances 

 of dispersal, whilst a shrub, starting at the same time, is getting 

 one, and may get from ten to thirty for the single opportunity 

 offered to a tree. It is thus evident in the first place that the 

 chance of rapid dispersal to a distance is much greater for the 

 herb, and in the second that the chance of forming a new species, 

 by whatever method it may be evoh^ed, is also much greater in 

 a given time. 



It must, however, be clearly understood that dispersal is 

 chiefly conditioned by the barriers which have already been dis- 

 cussed. Though the Compositae, for example, developed into a 

 herbaceous type, and though they developed a firstrate mechan- 

 ism for dispersal, they would not be so widespread and abundant 

 to-day were it not that the north temperate regions of the world 

 were largely cleared of forest by the ice in the glacial period, that 

 large areas became more open on account of desiccation of 

 climate, and that they were enabled to spread widely by the 

 development, often in comparatively recent periods, of the great 

 mountain chains which form an almost continuous track leading 

 over a very great proportion of the world, upon which they were 

 able to move above the limit of the forest, and often aided by 

 the formation of landslips (p. 37). One can clearly see that had 

 the world remained comparatively flat, and covered by forest, 

 to the present time, the Compositae to-day might be little more 

 widespread and abundant than say the Dijisacaceae. 



One may thus point to the development of herbaceous habit, 

 with the capability of living in open ground exposed to the sun, 

 as an ecological featiu'e which has made possible the compara- 

 tivel}' rapid and extensive spread of certain families, the spread 

 being accompanied by a correspondingly rapid development of 

 new forms, whether species or genera. But that rapid and wide 

 spread was only rendered possil^le by the incoming of certain 

 physical conditions to which these plants proved suited. It is 

 quite possible, if not probable, that these families, and even the 

 herbaceous type suited to open ground, are really very ancient, 

 but were confined to small localities, and never able to spread 

 widely, till the new conditions rendered it possible. There is 



