CLASSIFICATION. 57 



period to the present day, a lapse of many thousand years, 

 and through countless generations. When individuals of the 

 same brood differ in any respect, they are termed varieties ; for 

 example, one may be more exposed to the light, and become 

 brighter coloured ; or it may find more abundant food, and grow 

 larger than the rest. Should these peculiarities become perma- 

 nent at any place, or period, should all the specimens on a 

 particular island or mountain, or in one sea, or geological forma- 

 tion, differ from those found elsewhere, such permanent variety 

 is termed a race ; just as, in the human species, there are white 

 and coloured races. The species of some genera are less subject 

 to variation than others ; the nuculcE, for example, although very 

 numerous, are always distinguishable by good characters. Other 

 genera, like ammonites, terebratula, and tellina, present a most 

 perplexing amount of variation, resulting from age, sex, supply 

 of food, variety of depth, and of saltness in the water. And 

 further, whilst in some genera every possible variety of form 

 seems to have been called into existence, in others only a few, 

 strikingly distinct forms, are known. 



Genera are groups of species, related by community of struc- 

 ture in all essential respects. The genera of bivalves have been 

 characterised by the number and position of their hinge-teeth ; 

 those of the spiral univalves, by the form of their apertures; 

 but these technical characters are only valuable so far as they 

 indicate differences in the animals themselves. 



Families are groups of genera, which agree in some more 

 general characters than those which unite species into genera. 

 Those which we have employed are mostly modifications of the 

 artificial families framed ^by Lamarck, a plan which seemed more 

 desirable, in the present state of our knowledge, than a subdi- 

 vision into very numerous families, without assignable characters. 



The orders and classes of mollusca have already been referred 

 to ; those now in use are all extremely natural. 



It has been sometimes asserted that these groups are only 

 scientific contrivances, and do not really exist in nature ; but 



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