Dispersal of Herbaceous A agio sperms. 581 
distributed over the earth than is the present one, and which possessed very 
few herbaceous Dicotyledons. 
The predominance of woody plants on oceanic islands was apparently 
noted first by Darwin, but has also been emphasized by Hooker, Hemsley, 
and others. The only attempt to explain this phenomenon, so far as the 
writers have noted, is the one made by Darwin. In his discussion of the 
inhabitants of oceanic islands ( 4 , p. 413) he remarks that ‘islands often 
possess trees or bushes belonging to orders which elsewhere include only 
herbaceous species ; now trees, as Alph. de Candolle has shown, generally 
have, whatever the cause may be, confined ranges. Hence trees would be 
little likely to reach distant oceanic islands ; and an herbaceous plant, which 
had no chance of successfully competing with the many fully developed 
trees growing on a continent, might, when established on an island, gain an 
advantage over other herbaceous plants by growing taller and taller and 
overtopping them. In this case, natural selection would tend to add to the 
stature of the plant, to whatever order it belonged, and thus first convert it 
into a bush and then into a tree.’ But if trees really find it so difficult to 
reach distant islands, it seems strange that the flora of these regions should 
so universally be woody in its character. That the difficulty of migration 
among arborescent forms has been somewhat over-emphasized is indicated 
by the fact that such islands as the Azores and Bermuda, which we believe 
to have but recently received their flora, or at least the great bulk of it — 
and such others as Krakatoa, where we know the vegetation is but newly 
arrived — are almost or quite as well supplied with woody plants as are the 
lands from which they have derived their flora. Neither is there anatomical 
evidence that a tree has ever arisen from an increasingly vigorous herb. 
Herbaceous plants often grow rankly under very favourable circumstances, 
and may thus attain a woody zone of considerable width ; but although 
the outer portion of this zone may be composed of wood which is quite 
normal, the very reduced condition of vessels, rays, and parenchyma, charac- 
teristic of the ordinary herbaceous stem, may still be seen in that part of 
the vascular ring next the pith, and furnishes a clear proof that these plants 
are indeed overgrown herbs. Such instances, however, are rare, and prac- 
tically all plants which are typically woody display a normal and unreduced 
type of xylem structure next the pith. Also, these ‘ woody herbs ’ always 
retain their characteristic ability to produce flowers and seeds in a very 
short time. The advantages of this ability are so obvious that it is hard to 
imagine how it could ever have been lost, as lost it must have been if trees, 
none of which reach maturity so early, even on oceanic islands, have been 
derived from herbs. There is no information available to the effect that 
herbs introduced into oceanic islands tend to become more robust. On the 
contrary, animals and plants in such localities often grow smaller than the 
continental form. Herbs are the dominant type of vegetation to-day, and 
R r 2 
