meaning of nuts--Edible or attractive fruits--The colours of
flowers--Modes of securing cross-fertilisation--The
interpretation of the facts--Summary of additional facts
bearing on insect fertilisation--Fertilisation of flowers by
birds--Self-fertilisation of flowers--Difficulties and
contradictions--Intercrossing not necessarily
advantageous--Supposed evil results of close interbreeding--How
the struggle for existence acts among flowers--Flowers the
product of insect agency--Concluding remarks on colour in nature



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