x PREFACE. 



plainly stated. It would be invidious to mention any special sources 

 to which I have been indebted for aid in the production of the 

 present work : the field is so vast, that one must needs gather 

 details from the stores of many workers : but I cannot refrain from 

 expressing my indebtedness to the works of the late distinguished 

 author of the theory of " Natural Selection," and to those of Professor 

 Huxley and of Sir John Lubbock. My best thanks are due to the 

 latter for his kind permission to use several illustrations from his 

 interesting work on the relations between insects and flower- fertilisa- 

 tion. The illustrations as a whole will, I trust, be found to materially 

 assist the comprehension of the most important points discussed in 

 the various chapters. 



A. W. 



EDINBURGH : October 1882. 



