DESULTORY SKETCHES IN NATURAL HISTORY. 51 



is based on very distinct ordinal sub -types of form. The Swallow, 

 framed on the same model as every singing-bird, retains all the very 

 numerous peculiarities of structure observable throughout the ex- 

 ceedingly extensive group Cantores (Nobis) ; and neither in its 

 skeleton, digestive nor vocal organs, &c., presents any essential dif- 

 ference from a Sparrow, Robin, or Tree-creeper, from which it 

 varies only in minor adaptive modifications, such as the mere rela- 

 tive length of limbs, or the degrees of development of parts common 

 to all. The Swift presents not a single one of those characters, 

 but differs most materially in the structure of its whole skeleton and 

 entire anatomy ; its vocal apparatus, as in all the rest of its group 

 (the Strepitores, Nobis), not being complicated by peculiar muscles, 

 the function of which is to inflect the voice (as in every member of 

 the group to which the Swallow appertains), it can only utter a 

 discordant scream, while the Swallow modulates the tone of its 

 voice, and sings. But, without entering further into a specification 

 of internal distinctions, the exterior anatomy of the Swift and 

 Swallow, even to the structure of any single feather, or as observ- 

 able in the conformation of the bill and feet, in the number of tail- 

 feathers (which, in the group to which the Swallow belongs, is 

 invariably twelve ; while in the Swifts, as in many other Strepi- 

 tores, there are only ten), in short, in every imaginable particular 

 that can be supposed to indicate affinity or physiological proximity, 

 these two analogous genera have no better claim to rank in the 

 same order of the class of birds, than the Whale has, in consequence 

 of its external resemblance to a fish, to be included in the class of 

 fishes. We are fully prepared to state the veritable afiinities of 

 both the Swift and Swallow, but it would be out of place to do so 

 on this occasion. 



Having now, we trust, sufficiently elucidated the nature of the 

 systematic relations of animals in general, and controverted the pre- 

 valent notion that all groups are, to a certain extent, arbitrary, we 

 will proceed to our task of extricating various genera from the en- 

 tanglement of analogy J to assign them a position in accordance with 

 their intrinsical affinity ; and recur at length to the consideration of 

 the small group specified at the commencement of this article, as 

 one wherein the dental characters — so important generally, as indi- 

 cative of the affinities of Mammalia — become a deceptive guide to 

 the systematist who would place unlimited confidence in the vali- 

 dity of that as of any other single character, considered without 

 reference to the rest of the organization. In the Carnivora of the 

 illustrious Cuvier, we recognise four primary sub- types of form, viz. 



