56 MANUAL OF THE MOLLUSCAo 



genetic (or Jiistological), and are of the highest importauce. 

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

 sequence. 



Analogies. Modifications relating only to peculiar 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 (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 mi- 

 metic resemblances have led to the association of widely dif- 

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

 represented geographically! 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, 

 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 thein to have descended 

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

 provision for preventing the blending of species, that hybrids are 

 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." 



t E. g, Aporrhais with strombus, and ancylus with pateUa. 



% Monoceros imbricatum and buccinum antarcticum take the place, in 

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

 solen 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. 



