VI PREFACE. 



Some of the investigations have been pubHshed 

 in detail in memoirs that will be found in the Phil. 

 Trans, of the Royal Society, the Trans, of the 

 Microscopical Society, and elsewhere. The very last 

 observations upon which I have been engaged have 

 been included in Lecture XII, and four of the 

 drawings in Plates XV, XVI, XIX and XX have 

 been prepai'ed to illustrate them. 



How far the experiment of introducing the results 

 of recent investigations into a little text book has 

 been successful I must leave others to judge. My aim 

 in this and other works has been not only to 

 teach facts, but to encourage students to educate 

 themselves for original enquiry in order that they 

 may add new facts to those already known, and thus 

 advance the department of natural knowledge they 

 liave selected -for their life's work. 



HI, Grosvexor Street, 

 Septemher, 1872. 



