Darwinism | Page 3

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

CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT
SPECIES AND THE USUAL STERILITY OF THEIR HYBRID
OFFSPRING
Statement of the problem--Extreme susceptibility of the reproductive
functions--Reciprocal crosses--Individual differences in respect to
cross-fertilisation--Dimorphism and trimorphism among plants--Cases
of the fertility of hybrids and of the infertility of mongrels--The effects
of close interbreeding--Mr. Huth's objections--Fertile hybrids among
animals--Fertility of hybrids among plants--Cases of sterility of
mongrels--Parallelism between crossing and change of
conditions--Remarks on the facts of hybridity--Sterility due to changed
conditions and usually correlated with other characters--Correlation of
colour with constitutional peculiarities--The isolation of varieties by
selective association--The influence of natural selection upon sterility
and fertility--Physiological selection--Summary and concluding
remarks

CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS
The Darwinian theory threw new light on organic colour--The problem
to be solved--The constancy of animal colour indicates utility--Colour
and environment--Arctic animals white--Exceptions prove the
rule--Desert, forest, nocturnal, and oceanic animals--General theories
of animal colour--Variable protective colouring--Mr. Poulton's
experiments--Special or local colour adaptations--Imitation of
particular objects--How they have been produced--Special protective
colouring of butterflies--Protective resemblance among marine
animals--Protection by terrifying enemies--Alluring coloration--The
coloration of birds' eggs--Colour as a means of recognition--Summary
of the preceding exposition--Influence of locality or of climate on
colour--Concluding remarks

CHAPTER IX
WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration--Warning colours
among insects--Butterflies--Caterpillars--Mimicry--How mimicry has
been produced--Heliconidae--Perfection of the imitation--Other cases
of mimicry among Lepidoptera--Mimicry among protected groups--Its
explanation--Extension of the principle--Mimicry in other orders of
insects--Mimicry among the vertebrata--Snakes--The rattlesnake and
the cobra--Mimicry among birds--Objections to the theory of
mimicry--Concluding remarks on warning colours and mimicry

CHAPTER X
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
Sex colours in the mollusca and crustacea--In insects--In butterflies and
moths--Probable causes of these colours--Sexual selection as a

supposed cause--Sexual coloration of birds--Cause of dull colours of
female birds--Relation of sex colour to nesting habits--Sexual colours
of other vertebrates--Sexual selection by the struggles of males--Sexual
characters due to natural selection--Decorative plumage of males and
its effect on the females--Display of decorative plumage by the
males--A theory of animal coloration--The origin of accessory
plumes--Development of accessory plumes and their display--The
effect of female preference will be neutralised by natural
selection--General laws of animal coloration--Concluding remarks

CHAPTER XI
THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND
PURPOSE
The general colour relations of plants--Colours of fruits--The 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

CHAPTER XIII
THE GEOLOGICAL EVIDENCES OF EVOLUTION
What we may expect--The number of known species of extinct
animals--Causes of the imperfection of the geological
record--Geological evidences of evolution--Shells--Crocodiles--The
rhinoceros tribe--The pedigree of the horse tribe--Development of
deer's horns--Brain development--Local relations of fossil and living
animals--Cause of extinction of large animals--Indications of general
progress in plants and animals--The progressive development of
plants--Possible cause of sudden late appearance of
exogens--Geological distribution of insects--Geological succession of
vertebrata--Concluding remarks

CHAPTER XIV
FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION
AND HEREDITY
Fundamental difficulties and objections--Mr. Herbert Spencer's factors
of organic evolution--Disuse and effects of withdrawal of natural
selection--Supposed effects of disuse among wild animals--Difficulty
as to co-adaptation of parts by variation and selection--Direct action of
the environment--The American school of evolutionists--Origin of the
feet of the ungulates--Supposed action of animal intelligence--Semper
on the direct influence of the environment--Professor Geddes's theory

of variation in plants--Objections to the theory--On the origin of
spines--Variation and selection overpower the effects of use and
disuse--Supposed action of the environment in imitating
variations--Weismann's