CONTENTS 
xi 
plants—The same in animals—Uses of tails—Of the horns of deer 
Of the scale-ornamentation of reptiles—Instability of non-adaptive 
characters—Delbceuf’s law—No “specific” character proved to he 
useless—The swamping effects of intercrossing- Isolation as prevent¬ 
ing intercrossing—Gulick on the effects of isolation—Cases in which 
isolation is ineffective ..... Pages 126-151 
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 u nong 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 ..... 152-186 
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 experi¬ 
ments—Special or local colour adaptations- Imitation of particular 
objects — How they have been produced — Special protective colouring 
of butterflies — Protective resemblance among marine animals — Pro¬ 
tection 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 .... 187-231 
