22, NEW ZEALAND PLANTS. 
go to the wall. Nor, in most cases, can the plant escape from its 
surroundings. It is bound to its dwelling-place, where, or in the 
immediate neighbourhood—in the case of the higher plants—it must 
remain. But, as a member of a plant-association, its history has 
been that of a portion of the association; it has, in fact, become a 
member of the association as the result of rigorous selection. Its 
ancestor, as a seedling from a chance seed which fell upon a spot 
where the conditions allowed it to germinate, would depend for its 
subs stence upon its power to make the best of the circumstances. 
Were other better-equipped plants present, the plant in question 
would assuredly be wiped out in the struggle for existence. As 
time went on, the plants with which it was associated, together with 
itself and its brethren, would modify the environment for the better 
or for the worse, so far as the plant in question was concerned, and 
its power of survival might quite likely depend altogether upon its 
inherent plasticity. So with the changing years, as generation after 
generation of the plant held its own and its fellow-plants of other 
species were slowly being winnowed by the sieve of selection, a com- 
munity specially suited to the conditions offered would finally be 
segregated. 
The permanence of such a plant-community would depend partly 
upon the rapidity of the changes brought about by the plants them- 
selves, such as the ‘“‘ exhaustion” of the soil or the increase in its 
amount of vegetable matter, and partly upon topographic changes 
caused, for example, by erosion, or sinking or rising of the land- 
surface. If the topographical change was extremely slow, appa- 
rent stability would come and a plant-community of wide range 
governed chiefly by climate be the result, such as the rain-forest or 
the tussock-grassland of New Zealand. If, however, rapid changes 
of the land-surface were in progress there would be many formations 
apparently of a temporary character, such as those of rocks, of 
stony river-beds (fig. 12), of steep gullies (fig. 25), of debris-slopes 
(figs. 51, 68, 70), and so on; but which, if wiped out by topographic 
change in one place, would reappear in another so long as similar 
changes in the land-surface went on. 
From the above it is clear that there are two kinds of formations 
to be considered—the one, apparently fixed, which may be called 
_a climax formation, and the other, apparently of short duration, 
which, following Dr. C. B. Crampton, of H.M. Geological Survey, 
may be called a migratory formation. 
