196 NEW ZEALAND PLANTS. 
is interesting to note that with regard to the species common to 
the lowlands and to the high mountains the number fitted for long- 
distance travel by aid of birds or wind is greater, 20 per cent. being 
suitable for carriage by birds and 28 per cent. for carriage by wind. 
Water, either of streams, of the sea, or the run-off water of 
slopes, functions strongly with regard to carriage of seeds. In the 
mountains snow-avalanches and land-slides must be of considerable 
moment. 
With regard to methods of travel, it should be ever borne in mind 
that such movement of species must be extremely slow, and that 
the plant has time unthinkable at its disposal. Also, with regard 
to the period since there have been plant-formations on the earth 
with their members close together, it is not a movement of indi- 
viduals, except in the association, but a movement of the whole 
association that has to be considered. It resolves itself, in fact, 
into that most fundamental of biological causes—the struggle for 
existence ; in this case, too, a struggle between communities well 
equipped to hold their own, and where the weak are well protected. 
In considering the possibility of seeds travelling long dis- 
tances, the arrival at the destination is merely a preliminary to 
permanent settlement. The seed has first to germinate; then the 
young plant has to grow; and finally it is exposed to competition 
with its neighbours. If these things are borne in mind, the prospect 
of a chance seedling gaining a footing on the soil is small indeed: 
how small is well illustrated in the case of the primitive plant- 
associations of New Zealand. Bombarded with the seeds of foreign 
plants for more than one hundred years, not one of these aliens has 
become a member of primeval forest, herb-field, grassland, shrub- 
land, or bog! 
All species have a more or less discontinuous distribution—.e., 
there are always areas, great or small, as the case may be, where 
the species does not occur. ‘This is the consequence of the species 
being arranged in associations. -If, however, the associations in 
which a species occurs extend with breaks between them through- 
out a region, the distribution of that species would be called 
“ continuous.” Between such continuity and a species occurring in 
very few localities there is every degree of transition. The cases 
about to be noted are where the discontinuity is excessive. Only a 
few examples are given, but the phenomenon is quite common. 
