252 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
led to the association of groups of similar species into 
genera, has given rise to the combination of allied genera 
into higher groups, which are termed Families. It is 
obvious that the definition of a family, as a statement of 
the characters in which a certain number of genera agree, 
is another morphological abstraction, which stands in the 
same relation to generic, as generic do to specific abstrac- 
tions. Moreover, the definition of the family is a statement 
of the plan of all the genera comprised in that family. 
The family of the Northern crayfishes is termed 
Potamobiide ; that of the Southern crayfishes, Par- 
astacide. But these two families have in common all 
those structural characters which are special to neither ; 
and, carrying out the metaphorical nomenclature of the 
zoologist a stage further, we may say that the two form 
a Tribe—the definition of which describes the plan which 
is common to both families. 
It may conduce to intelligibility if these results are put 
into a graphic form. In fig. 66, A. is a diagram represent- 
ing the plan of an animal in which all the externally 
visible parts which are found, more or less modified, in 
the natural objects which we call individual crayfishes 
are roughly sketched. It represents the plan of the 
tribe. B. is a diagram exhibiting such a modification 
of A. as converts it into the plan common to the whole 
family of the Parastacide. C. stands in the same re- 
lation to the Potamobiide. If the scheme were thoroughly 
worked out, diagrams representing the peculiarities of 
