14 



INTRODUCTION. 



the theory, because it collects, under one point of view, and 

 gives a rational explanation of, many apparently independent 

 classes of facts. 4 



4 In treating the several subjects 

 included in the present and my other 

 works 1 have continually been led to 

 ask for information from many zoolo- 

 gists, botanists, geologists, breeders of 

 animals, and horticulturists, and I 

 have invariably received from them 

 the most generous assistance. With- 

 out such aid I could have effected 

 little. I have repeatedly applied 

 for information and specimens to 



foreigners, and to British merchants 

 and officers of the Government re- 

 siding in distant lands, and, with the 

 rarest exceptions, I have received 

 prompt, open-handed, and valuable 

 assistance. I cannot express too 

 strongly my obligations to the many 

 persons who have assisted me, and 

 who, I am convinced, would be 

 equally willing to assist others in 

 any scientific investigation. 



