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 considefable 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 1s 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 

