954: FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
(313.) Let us for the present suppose, theoretically, 
that each of these columns is a circular group. One of 
the tests, therefore, of such a group is, that it finds its 
contents represented in all others ; hence, the results in 
the present instance completely confirm what has just 
been advanced: we find the cetacea, or whales, repre- 
senting the fishes, the hippopotamus, the ostrich, and the 
Neuropterous dragon-flies. By this series we know, 
therefore, to demonstration, that the analogies are cor- 
rect, because they are the result, not of a studied adapt- 
ation, but arise, as it were incidentally, from following 
closely the line of affinity in each column; the affinities 
being expressed perpendicularly, and the analogies hori- 
zontally. 
(314.) We are now to consider the sucror1aL type of 
form: this corresponds with the tenuirostral type among 
perching birds, the grallatorial in the orders of that 
class, the gliriform among quadrupeds, and the onisci- 
form and vermiform in the class of insects. We shall, 
however, designate all these under the common name of 
suctorial, because it is more generally applicable to the 
habits of the animals here alluded to than to any other. 
One of the chief peculiarities of this type is, that the 
food is imbibed by suction ; a mode of nourishment 
which is, of course, accompanied by many remarkable 
deviations from the structure of all other types. These 
are always the smallest in point of size, the most feeble 
and defenceless in structure, and the most defective in 
the organs of mastication. In all these characters, the 
suctorial stands in direct opposition to the natatorial 
type. In such as belong to the vertebrated circle, the 
feet are always fully developed ; for these animals are 
peculiarly active, and enjoy, in a remarkable degree, 
the power of running and of leaping. The suctorial 
form is also widely different from the natatorial in other 
respects ; there is a great length or attenuation of the 
body, the head is always very small, generally pro- 
longed into a pointed snout, and the mouth, as adapted 
for sucking, is uncommonly small: in some few in- 
