Vlll PREFACE. 



ever, that he has succeeded in laying before his readers, 

 not a mere mass of undigested facts, but something like 

 an orderly and systematic review of the main points 

 required to be known by the student. The Author is 

 conscious of many imperfections in his plan, and also in 

 the execution of his plan. The subject, however, is so 

 extensive and so constantly changing that he can reason- 

 ably claim some indulgence, if the brief leisure-time of a 

 busy life has not enabled him in every respect to keep 

 abreast of the latest discoveries. Such defects as there 

 may be, are, it is hoped, of such a nature as not to diminish 

 the value of the work for ordinary students. 



Amongst the sources upon which the Author has mainly 

 drawn, it is, perhaps, invidious to mention one more than 

 another. He feels, however, bound to acknowledge with 

 gratitude the very great assistance which he has derived 

 from the various works of Professor Huxley. 



EDINBURGH : 



Nov. 2, 1869. 



