46 MANUAL OF THE MOLLUSCA. 
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 ayail, 
‘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 neryous systems. 
Species. All the specimens, or individuals, which are so 
much alike that we may reasonably believe them to haye 
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 nucwle, for example, 
although yery numerous, are always distinguishable by good 
characters. Other genera, hike ammonites, terebratula, and tel- 
lina, 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. 
Genera are groups of species, related by community of struc- 
ture in all essential respects. The genera of bivalyes have been 
characterised by the number and position of their hinge teeth ; 
those of the spiral univalyes, 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 haye 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 yery numerous families, without assignable 
characters. 
