vi PREFACE 



seventh, eighth, and ninth essays being concerned with 

 the value of the visible surface of animals in the struggle 

 for life, it was considered appropriate to include under 

 the tenth head, and to illustrate by many examples, a 

 comprehensive classification of the various uses which 

 external colouring and appearance may subserve. 



It will, I think, be realized that, although the separate 

 essays were delivered as addresses or published on various 

 occasions and at very different dates, they are the expres- 

 sion of a continuous line of thought, and therefore fall 

 together as naturally as if they had been written at one 

 time, for the purposes of the present volume. 



A certain amount of overlap is necessary in essays 

 dealing with closely related subjects. Any inconvenience 

 from this cause and the scattered use of examples, in- 

 evitable in an essay, will be removed by consulting the 

 Appendix and the analytical index, to the preparation of 

 which much time and labour have been devoted. 



Wherever the progress of knowledge has led to modifi- 

 cation of statement or conclusion the necessary alterations 

 have been made. These are in the form of footnotes 

 whenever the importance and interest of the advance is 

 such as to call for prominence. In other cases I have not 

 hesitated to alter the text. Whether right or wrong, the 

 arguments and conclusions in this volume of essays 

 represent my views at the present moment. 



It will be observed that the style of the third essay 

 differs from that of all the others. This is because it is 

 the revised shorthand record of an address, spoken not 

 read. The seventh (hitherto unpublished) and ninth 

 addresses were also spoken, but the corresponding essays 

 were written subsequently from the notes. They there- 



