CLASSIFICATION. 45 



resemble tlie simpler orders of their own sub-kingdom, but not 

 tbose of any other group. 



These are the views of Professor Owen — the successor of 

 Hunter — by whom it has been most clearly shown and stead- 

 fastly maintained, that the " unity of organisation" manifested 

 by the animal world results from the design of a Supreme In- 

 telligence, and cannot be ascribed to the operation of a mechani- 

 cal "law." 



CLASSIFICATIOX. 



The objects of classification are, first, the convenient and in- 

 telligible arrangement of the species;* and, secondly, to afford 

 a summary, or condensed exjDOsition, of all that is kno"v\Ti re- 

 specting their structure and relations. 



In studying the shell-fish we find resemblances of two kinds. 

 First, agreements of structure, form, and habits ; and, secondly, 

 resemblances of form and habits without agreement of struc- 

 ture. The first are termed relations of ajfinity ; the second, of 

 analogy. 



Affinities may be near, or remote. There is some amount of 

 affinity common to all animals ; but, like relationships amongst 

 men, they are recognised only when tolerably close. Eesem- 

 blances of structure which subsist from a very early age are 

 presumed to imply original relationship ; they have been termed 

 genetic (or It istological), and are of the highest importance. Those 

 which are suj)erinduced at a later period are of less consequence. 



Analogies. Modifications relating only to peculiar habits are 

 called adaptive; or teleological, from their relation to final 

 causes, t A second class of analogical resemblances are purely 

 external and illusive ; they have been termed mimetic [Strick- 

 land), and, by their frequency, almost justify the notion that a 

 certain set of forms and colours are repeated, or represented in 

 every class and family. In all artificial arrangements, these 

 mimetic resemblances have led to the association of widely dif- 

 ferent animals in the same groups. J Particular forms are also 

 represented geographically§ and geologically, |1 as well as sys- 

 tematically. 



* At least 20,000 recent, and 16,000 fossil species of molluscous animals are known. 



t For example, tlie paper nautilus, from its resemblance to carinaria, was long sup- 

 posed to be the shell of a nucleobranch, parasitically occupied by the " ocythoe.^'' 



X e. g. Aporrhdis with stromUis, and ancylus with patella. 



§ 31o)ioceros imhricatum and bitccinvm antarcticum take the place, in South 

 America, of our conomon whelk and purple, and solen gladiolus and solen amcricanus 

 of our solen siliqua and ensis. 



I The frequent recurrence of similar species in successive strata may lead beginners 

 to attribute too much to the influence of time and extemai circumstances ; but sui-lf 

 impressions disappear witi further experience. 



