228 ‘ B. NATURAL HISTORY. 
system at present includes, the definitions of our groups will rest 
upon single characters only, and. the history of the origin of those 
characters will be the history of the origin of the groups. 
It is the proper discrimination of the relative values of these 
single characters which in our estimation determines the “natu- 
ralness” of a system; and the principle on which such discrimina- 
tion reposes is the key to that perplexing question which often 
renders the conclusions of naturalists so different in appearance, 
while the objects of their investigations are the same. But by mis- 
using “technical” or single characters — that is, by misinterpreting 
their values —the most erroneous approximations may be made, 
and systems constructed which well deserve the term “ artificial” 
applied to them by those who, in their search for the “natural” 
system, are opposed to the use of “technical” characters. Per- 
haps the best known example of this misuse is to be found in the 
Linnean system of botany, where the value of the numbers of 
' stamens and pistils in determining affinity was placed much too 
high. Though this system has been utterly abandoned, yet Lin- 
nus’s characters are still of great importance in a lower grade 
of relations. 
As the number of primary groups of the animal kingdom is but 
small, I will commence with the principle on which all subordinate 
divisions may be distinguished, and their value ascertained. 
I. Given primary divisions, and given that such divisions pre- 
sent in some members greater resemblance (or unity of minor 
characters) to members of other primary divisions, and in other 
members especial diversity from the same,— the primary subdivi- 
sions of said first divisions are those which express the successional 
degrees of resemblance to or difference from the other divisions of 
first rank. , : 
-II. Given primary subdivisions, their subdivisions of first rank 
are estimated, as in Prop. I, by reference to the characters pre- 
sented by their extremes of likeness to or diversity from the mem- 
bers of the other primary subdivisions. The value of characters 
of the groups contained in each of last grades mentioned to be 
determined by the same test. 
For primary divisions, in Prop. I., might be read class ; for-pri- 
mary subdivision, order ; and for subgroup of the latter, family. 
The same principle applies to genera, which is expressed in [rop. 
VI. of a series designed to render clear the basis of the theory of 
evolution, published in a “Monograph of the Cyprinide of Penn- 
