Effect of struggle for existence under unchanged conditions--The
effect under change of conditions--Divergence of character--In
insects--In birds--In mammalia--Divergence leads to a maximum of
life in each area--Closely allied species inhabit distinct
areas--Adaptation to conditions at various periods of life--The
continued existence of low forms of life--Extinction of low
types among the higher animals--Circumstances favourable to the
origin of new species--Probable origin of the dippers--The
importance of isolation--On the advance of organisation by
natural selection--Summary of the first five chapters



CHAPTER VI

DIFFICULTIES AND OBJECTIONS


Difficulty as to smallness of variations--As to the right
variations occurring when required--The beginnings of important
organs--The mammary glands--The eyes of flatfish--Origin of the
eye--Useless or non-adaptive characters--Recent extension of the
region of utility in plants--The same in animals--Uses of
tails--Of the horns of deer--Of the scale-ornamentation of
reptiles--Instability of non-adaptive characters--Delboeuf's
law--No "specific" character proved to be useless--The swamping
effects of intercrossing--Isolation as preventing
intercrossing--Gulick on the effects of isolation--Cases in
which isolation is ineffective