Vlil PREFACE. 



desire to learn the outlines of the Science. In carrying 

 out this object, it is unnecessary for the Author to re- 

 mark that he does not lay any claim to originality. He 

 trusts, however, 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 reasonably 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 Pro- 

 fessor Huxley. 



EDINBURGH, November 2, 1869. 



