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 Five great divisions of the animal kingdom might be established, and named: 
Protozoa, Actinozoa, Ectinozoa, Malacozoa, Spondylozoa. We are too indifferent to the 
introduction of new names to propose these except in a note. Under Ectinozoa (from 
éxreivw, 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, 
