254 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 j 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 j the affinities 

 being expressed perpendicularly, and the analogies hori- 

 zontally. 



(314.) We are now to consider the SUCTORIAL 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- 



