200 SPENCER MOORE [September 



supposition deals with secular changes, that is with conditions entirely 

 outside the limited range of our experience, speculations on the subject 

 cannot be said to be quite conclusive. The fact we have to recognise 

 is that migration has taken place, whatever may have been the agency 

 or agencies whereby it was effected. 



Now, the most successful migrants should be herbs, for the seeds 

 of herbaceous annuals falling upon favourable soil will rapidly 

 germinate, and the seedlings will run through their life-history in a 

 season lasting only a few weeks. So, too, free-seeding biennials and 

 perennials will take possession of an unoccupied area, and produce 

 offspring soon ready in their turn to extend the range of the species 

 whenever occasion offers. Far otherwise is it with shrubs and trees, 

 which require several years before they bear seeds. Competition, too, 

 between trees and shrubs will be much keener than between herbs ; 

 for each of the former must have a considerable space for the support 

 of its assimilating organs; their area also will be limited by such a 

 condition as depth of soil, and they are liable to destruction by storms. 

 Moreover, unoccupied spaces are left between them, and here herbs can 

 flourish. And when it is remembered that the stepping-stones, as it 

 were, which have been made use of in the transport of plants across 

 the tropics — the mountain-ranges, that is to say — are especially 

 adapted to herbs, many of them living above the regions of trees and 

 shrubs, we see how great an advantage in migration has been enjoyed 

 by herbaceous plants over woody ones. 



We come now to the next point, which is, that while in the north 

 part of the northern hemisphere the proportion of herbs to shrubs and 

 trees is so large as to justify our calling this portion of the globe a 

 herbaceous zone, the south part of the southern hemisphere, where it 

 is not occupied by the ocean or by glaciated land, comes for the most 

 part within what I shall term a dendritic zone, meaning by this a zone 

 where woody vegetation predominates over herbaceous. New Zealand, 

 temperate Australia, South Africa, the greater part of extra-tropical 

 South America are all dendritic lands. Given, therefore, opportunities 

 of transport from either hemisphere into the other under conditions 

 similar or approximately similar to those now existing, and herbs 

 being better adapted to transport than woody plants, the probabilities 

 are that the preponderating trend of migration will be from north to 

 south, and this without any inherent superiority in the northern flora 

 due to competition in the largest land-area of the world, or to any 

 other cause whatsoever. 



" But," one fancies an objector saying, " consider how large a 

 number of northern species have passed over into the southern hemi- 

 sphere, and how few and far between, and even then how limited in 

 their range north of the Equator, are the southern types which have 

 succeeded in gaining a foothold in the northern hemisphere." But 

 this statement assumes our possession of more knowledge than is at 



