36 INTRODUCTION. 



form an unbroken chain ; that without any sudden leaps, all hang 

 together by insensible transitions. But the breaks in the chain 

 have not been filled up by later observation, rather have new and 

 previously unrecognised deviations from it been discovered. It is 

 not a ladder with uniformly ascending rounds, but rather a net 

 which may afford us a conception of the multifarious connexions 

 and the various affinities according to which nature has arranged 

 her products. 



We have already remarked that the vertebrate animals ascend 

 to the highest grade of perfection of organisation : of them, there- 

 fore, we may properly consider the different classes last. 



In treating of the Animal Kingdom we shall not make use of 

 CUVIER'S distribution of it into four divisions, further than as a 

 guiding idea. The Infusories (exclusive of the Rotatories and 

 others, which were joined to them on account of their minuteness 

 alone) appear to form a distinct group, or at least do not indicate 

 the radiating form by which Polyps and others of the lower animals 

 are distinguished. We make, therefore, for these simplest animal 

 existences a distinct Division, naming them, after the example of 

 other authors, Protozoa. Their form is round or oblong, often not 

 rigorously determined, but variable during life 1 . 



1 Five great divisions of the animal kingdom might be established, and named: 

 Protozoa, Actinozoa, Ectinozoa, Mcdacozoa, Spondylozoa. We are too indifferent to the 

 introduction of new names to propose these except in a note. Under Ectinozoa (from 

 tttTdvu, extendo) we understand those animals in whose organisation the elongated type 

 prevails: they nearly agree with the Articulata. The other names have in part been 

 used already, and require no further explanation. 



