8 INTRODUCTORY LECTURE. 



rise and diverge. Every naturalist or physiologist is at liberty, of 

 course, to adopt any one of the characters that have been supposed 

 to divide the two kingdoms ; but the boundary, so defined, vvill be 

 artificial, and each different character will bisect the debateable ground 

 in a different latitude of the organic world. 



Animals and plants, then, are not two natural divisions, but are 

 specialised members, of one and the same great group of organised 

 beings. When a certain number of characters concur in the same 

 organism, its title to be regarded as a "plant" or an "animal" 

 may be readily and indubitably recognised; but there are very nu- 

 merous living beings, especially those that retain the form of nucleated 

 cells, which manifest the common organic characters, but without 

 the distinctive superadditions of either kingdom. Such organisms 

 are the DiatomacecB, Desmidice, Protococci , Volvocince, Vibriones, 

 Astasiceee, Thalassicolce, and Spongice ; all of which retain the cha- 

 racter of the organised fundamental nucleated cell, with comparatively 

 little change or superaddition. 



In a work treating expressly of the animal organisation I am un- 

 willing to include more than the zoologist may fairly lay claim to, and 

 I therefore look for certain combinations of characters as qualifications 

 for admission, although occasional illustrations of organic functions 

 w^ill be derived from the indeterminate organisms above mentioned. 



Thus if irritability of parts or locomotion of the w^hole be not 

 accompanied with mouth and stomach, it must be manifested by 

 proximate tissues of quaternary compounds, and the cell-walls and 

 fibres must consist of albuminous or gelatinous matter, or be soluble 

 in acetic acid. 



If some of the tissues of an organism are binary compounds, yet 

 if they include a stomach with a mouth, and are also associated either 

 with irritable parts or a power of locomotion, such organism will be 

 referred to the animal kingdom. 



When to a mouth and alimentary canal are superadded definite 

 muscular and nervous filaments, a heart, a breathing apparatus, and 

 generative organs, no doubt of the animality of the organism can 

 be entertained. 



Having thus laid down the grounds on which I have marked out 

 that higher division of organised bodies which is the proper subject 

 of the studies of the zootomist, I proceed next to oifer a few remarks 

 on the leading vai'ieties and grades of structure in the animal king- 

 dom, in so far as they are guides to its classification. 



Little useful progress can be made in Comparative Anatomy 

 without some knowledge of Zoology. Zoology is the key to the na- 

 ture and habits of tlie animals of which Zootomy unfolds the struc- 



