d the
effects of their primitive habitat through heredity. The 3-merous [sic]
perianth of grasses, the parts of the flower being in whorls, point to a
degradation from a sub-liliaceous condition."

Mr. Henslow informs me that he has long held these views, but, as far as
he knows, alone. Mr. Grant Allen, however, set forth a similar theory in
his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of
Flowers_ (chap. v.), where he develops it fully and uses similar
arguments to those of Mr. Henslow.]

[Footnote 156: H. Mueller gives ample proof of this in his _Fertilisation
of Flowers_.]

[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]

[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]

[Footnote 159: Mueller's _Fertilisation of Flowers_, pp. 448, 455. Other
cases of recent degradation and readaptation to insect-fertilisation are
given by Professor Henslow (see footnote, p. 324).]

[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]

[Footnote 161: _The Colour Sense_, chap. ix.]

[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]

CHAPTER XII

THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS

The facts to be explained--The conditions which have determined
distribution--The permanence of oceans--Oceanic and continental
areas--Madagascar and New Zealand--The teachings of the
thousand-fathom line--The distribution of marsupials--The
distribution of tapirs--Powers of dispersal as illustrated by
insular organisms--Birds and insects at sea--Insects at great
altitudes--The dispersal of plants--Dispersal of seeds by the
wind--Mineral matter carried by the wind--Objections to the
theory of wind-dispersal answered--Explanation of north
temperate plants in the southern hemisphere--No proof of
glaciation in the tropics--Lower temperature not needed to
explain the facts--Concluding remarks.

The theory which we may now take as established--that all the existing
forms of life have been derived from other forms by a natural process of
descent with modification, and that this same process has been in action
during past geological time--should enable us to give a rational account
not only of the peculiarities of form and structure presented by animals
and plants, but also of their grouping together in certain areas, and
their general distribution over the earth's surface.

In the absence of any exact knowledge of the facts of d