46 MANUAL OP THE MOLLtJSCA. 



In all attempts to cliaractcrise 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 

 condition 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 always barren ; and it is certain, in the case of shells, 

 that a great many kinds have not changed in form from the 

 tertiary 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 

 permanent at any place or period, — should all the specimens on 

 a particular island or mountain, or in one sea, or geological 

 formation, 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 tel' 

 Una, 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 pos- 

 sible variety of form seems to have been called into existence, in 

 others only a few, strikingly distinct forms, are known. 



Oenera 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 

 subdivision into very numerous families, without assignable 

 characters. 



