COXTEXTS xi 



plants — The same in animals — Uses of tails — Of the horns of deer — 

 Of the scnle-ornamentation of reptiles — Instability of non-adaptive 

 characters — Delbceiif's law — Xo "specific" character proved to be 

 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 



CHAPTEE YIl 



ON THE INFERTILITY OF CROSSES BET^YEEN 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 ^-mong plants — 

 Cases of the fertility of hybrids and of the infertility of moiigi-els 

 — The effects of close interbreeding — Mr. Huth's objections — Fertile 

 hybrids among animals — Fertility of hybrids among plants — Cases of 

 sterility of mongrels — Parallelism beto'een 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 

 concludiuct remarks ..... 152-186 



CHAPTER VIII 



THE ORIGIN AND USES OF COLOI'R IN ANBL\LS 



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 colouriug — 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 



