origin within comparatively modern geological
epochs." (Loc. cit.) This is no doubt a correct conclusion. But it is
difficult to explain on Darwin's view alone, of alternating cold in the
two hemispheres, the preponderant migration from the north to the south.
He suggests, therefore, that it "is due to the greater extent of land
in the north and to the northern forms... having... been advanced through
natural selection and competition to a higher stage of perfection or
dominating power." ("Origin of Species" (6th edition), page 340; cf.
also "Life and Letters", II. page 142.) The present state of the Flora
of New Zealand affords a striking illustration of the correctness of
this view. It is poor in species, numbering only some 1400, of which
three-fourths are endemic. They seem however quite unable to resist the
invasion of new comers and already 600 species of foreign origin have
succeeded in establishing themselves.

If we accept the general configuration of the earth's surface as
permanent a continuous and progressive dispersal of species from the
centre to the circumference, i.e. southwards, seems inevitable. If an
observer were placed above a point in St George's Channel from which
one half of the globe was visible he would see the greatest possible
quantity of land spread out in a sort of stellate figure. The maritime
supremacy of the English race has perhaps flowed from the central
position of its home. That such a disposition would facilitate a
centrifugal migration of land organisms is at any rate obvious, and
fluctuating conditions of climate operating from the pole would supply
an effective means of propulsion. As these became more rigorous animals
at any rate would move southwards to escape them. It would be equally
the case with plants if no insuperable obstacle interposed. This implies
a mobility in plants, notwithstanding what we know of means of transport
which is at first sight paradoxical. Bentham has stated this in a
striking way: "Fixed and immovable as is the individual plant, there is
no class in which the race is endowed with greater facilities for the
widest dispersion... Plants cast away their offspring in a dormant state,
ready to be carried to any distance by those external agencies which we
may deem fortuitous, but without which many a race might perish from the
exhaustion of the limited spot of soil in which it is rooted." (Pres.
Addr.(1869), "Proc. Linn. Soc." 1868-69, pages lxvi, lxvii.)