INTRODUCTION. XIU 



The larger animals, especially sucli as are either 

 useful or dangerous to man, and with which, 

 therefore, most persons are familiar, differ so 

 much, in the adult condition, from the more con- 

 spicuous members of the vegetable kingdom, by- 

 reason of their powers of voluntary locomotion, as 

 also by the anatomical feature of possessing a 

 nervous system, that the present inquiry might 

 seem, at first sight, unnecessary and even ridicu- 

 lous. What resemblance is there, it might be 

 asked, between a bird and the tree on which it 

 perches, or a cow and the grass upon which it 

 feeds ? 



But the scientific naturalist is acquainted with 

 numerous aquatic organisms, most (though not all) 

 of these being of minute size, whose true position 

 whether in the animal or vegetable kingdom has, 

 of late years, afforded matter for abundant contro- 

 versy. Many of these creatures, while presenting 

 affinities to undoubted plants, are nevertheless 

 capable of executing movements which have 

 been frequently compared to those of animals. 

 Besides, there are other living beings, fixed dur- 

 ing the entire period of their lives, whose animal 

 natm^e is not likely to be called in question. Fm^- 

 ther, several animal forms, universally recognised 

 by naturalists as such, exhibit no traces of a ner- 

 vous system. 



The presence or absence of locomotive power, and 

 of a nervous system, will, therefore, in many cases, 



