56 MANUAL OF THE MOLLUSCA. 



genetic (or JiistologicaT), and are of the higliest importance. * 

 Those which are superinduced at a later period, are of less con- '\ 

 sequence. 1 



Analogies. Modifications relating only to pecidiar habits ' 

 are called adaptive ; or teleological, from their relation to final ' 

 causes.* A second class of analogical resemblances are purely ■. 

 external and illusive; they have been termed mimetic (St?'ick- ■ 

 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 mi- ' 

 metic resemblances have led to the association of widely dif- | 

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

 represented geographically J and geologically, § as well as sys- •: 

 tematically. | 



In all attempts to characterise groups of animals, we find, > 

 that in advancing from the smaller to the larger combinations, J 

 many of the most obvious external features become of less avail, | 

 and we are compelled to seek for more constant and comprehen- '• 

 sive signs in the phases of embryonic development, and the con- \ 

 dition of the circulating, respiratory, and nervous systems. 



Species. All the specimens, or individuals, which are so much 

 alike that we may reasonably believe them to have descended 

 from a common stock, constitute a species. It is a particular 

 provision for preventing the blending of species, that hybrids are j 

 always barren; and it is certain, in the case of shells, that a \ 

 great many kinds have not changed in form, from the tertiary ] 



* For example, the paper nautilus, from its resemblance to carinaria, was . 



long supposed to be the shell of a nucleobranche, parasitically occupied by the ] 



'' oci)thoe" j 



t E. g. Aporrhais with strombus, and ancylus with patella. j 



\ Monoceros imbricatum and buccimim antarcticimi take the place, in j 



South America, of our common whelk and purple, and solen gladiolus and j 



soleu americanus of our solen siliqua and ensis. ' 



\ The frequent recurrence of similar species in successive strata may lead ■ 



beginners to attribute too much to the influence of time and external circum- ' 



stances ; but such impressions disappear with further experience. - 



