PREFACE. vii 



readers, not a mere mass of undigested facts, but something 

 like an orderly and systematic review of the main points re- 

 quired 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 con- 

 stantly 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 Professor Huxley. 



EDINBURGH, November 2, 1869. 



PREFACE TO FIRST COMPLETE EDITION. 



IN issuing the first part of the present work in a second edition, 

 and in bringing out the second part, the Author has little to 

 add to what he has already said. 



The chief point upon which it may be desirable to say a few 

 words is, as to the object aimed at in the Introductory portion 

 of the work. The Introduction is intended to exhibit to the 

 student, in as brief a form as possible, the leading principles 

 of Zoological Science. These principles are of the highest 

 importance, and no adequate knowledge of Zoology can be 

 attained without their full comprehension. At the same time, 

 the principles in question depend, in many cases, upon data 

 which are only evolved during the systematic study of the sub- 

 ject. For this reason, it is not to be expected that the student 



