STABLE AND MIGRATORY PLANT FORMATIONS 13 
than secular geological mutations, are to be looked for in 
the secular evolution and migrations of animals, and in the 
increasing specialisation and competition of the various 
species of plants. As a single, oft-quoted example, we may 
point to the profound effects on vegetation which must have 
followed the first introduction of social herbivorous mammals. 
The existing natural vegetation of New Zealand has evolved 
without such complications. A comparison of the vegetation 
of New Zealand with that of Patagonia and Australia should 
reveal much of interest from this standpoint. 
By extensive changes such as those outlined above, parts 
of a tropical forest area may come to lie beyond their former 
districts of great rainfall, and the dominants of the forest 
vegetation must undergo degeneration, a desert may become 
partly habitable by grassland or forest, and an island flora 
may cease to preserve its isolation. Such marked alterations 
in climate or geography cause dislocations in the balance of 
relations between the various species of plants and animals 
and their surroundings, and chiefly in accordance with the 
rapidity of the transformation. In one direction it may lead 
to a keener struggle for existence, accompanied by extinction 
of these species less suited to the changed conditions ; in the 
other, with lessening competition and a wider dispersal of 
species under new conditions, it may lead to mutation and 
the establishment of new types. It is indeed conceivable 
that natural selection tends rather to the preservation of the 
type through restricting its area of migration to a certain 
defined habitat where only a certain defined mutation can 
reach maturity ; while a loosening of the reins, as it were, 
may allow of the colonisation of a wider range of habitats by 
distinct mutations with success. Further, as suggested by 
Kerner, the widening of the areas of distribution of related 
species may bring about an overlapping of such areas, leading 
possibly to crossing, and the formation of new types (16). 
In these ways, and probably in others not indicated, not 
only may the various types of vegetation succeed one another 
in a manner entirely different from the ordinary topographic 
types of succession, with which to some extent we are familiar, 
but new types of species and plant associations may evolve 
in accordance with the new conditions. 
