CONTENTS Xl 



plants — The same in animals — Uses of tails — Of tlie 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 prevent- 

 ing intercrossing — Gulick on the effects of isolation — Cases in which 

 isolation is ineffective ..... Pages 126-151 



CHAPTEE 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 diHerences 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 ..... 152-186 



CHAPTER VIII 



THE ORIGIN AND USES OF COLOUR IN ANUNIALS 



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 colom- 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 



