and Mhicralogij, ^ 5^5 
tiou of the smallest difference of uniformity ; and which are 
consequently more strictly united with each other ^ more closely 
contiguous^ than such as do not admit of being so treated. 
This agreement, however, is of little use ; because, when tried 
in practice, it is found that individuals, which agree exactly 
in all their characters but one, are of very rare occurrence, 
and that when the agreement in so many properties does take 
place, the one remaining property is also commonly the same, 
(the same form, the same colour, and so on,) and consequently 
that the individuals, being already uniform with each other, are 
not of such a kind as to render the discovery of their agreement a 
useful subject of inquiry. Nor does the preceding observa- 
tion lead to any natural historical conception of species, genus, 
or other classification ; and consequently, by the attainment of 
these objects, requires investigation to be farther prosecuted. 
An Individual may at once be a member of two Series. 
— Upon considering abstractly a series of forms, the only one, 
for example, of the tessular system, and a series of colours, 
say those of fluor-spar, it appears that an individual of the mi- 
neral kingdom may, in regard to its form, be a member of the 
first series, and at the same time, in regard to its colour, a 
member of the second. Hence a mineral that constitutes by 
its form a member in a series of individuals, which have their 
other relations the same, but differ in their form, may at the 
same time constitute a member in a series of individuals, which, 
agreeing in their other relations, differ in the shades of their co- 
lour. 
22. By this means the individuals of both series are brought 
under the conception of smallest difference, — When a number 
of individuals are distinguished solely by the differences of their 
forms, if these forms are members of the same series, the degree 
of difference among those individuals is the smallest that can pos- 
sibly take place, unless when the individuals are simply uni- 
form (20). Thus if a number of individuals are distinguish- 
ed merely in the differences of their colours, those colours be- 
ing members of one series ; the degree of difference is the 
smallest that can possibly take place in consistency with the 
individuals not being simply uniform (20.) Hence, when an 
individual is found which, considering the entire agreement of 
