PREFACE. IX 



correspondent that the question was asked 

 In no captious spirit, but with a real desire 

 for information, I could obtain no response 

 to my enquiry. I was therefore fain to 

 let the matter drop, naturally concluding 

 that if so eminent a scientist as Professor 

 Tyndall was unwilling to answer so plain 

 a question, the theory was not likely to be 

 true. 



After an interval of some years, I had 

 an opportunity of asking the same question 

 of Mr. Darwin himself, through the circum- 

 stance of his son and my son-in-law being 

 brother-officers in the Royal Engineers; 

 when he was kind enough to reply to my 

 question without delay. After a brief 

 correspondence, I give his final letter, 

 which was written about a year before his 

 lamented death, and which reads as 

 follows : 



"Down, Beckenham, Kent, Oct. 1, 1881. 



"Dear Sir, The secretary tubules in the mam- 

 mery glands are generally admitted to be of the 

 same nature or homologous with the glands 



over the whole skin. Even with mammals there 

 is some gradation in perfection as in the mammery 



