cet 
oe 
Natural History of the United States. 335 
ideal plan, structural type, order or rank, form, and diversity 
of details and adaptations, are ‘all the categories of relation- 
ip which exist among animals, as far as their structure is 
concerned.” 
_ There is a freedom in nature in the use of form, structure and 
differences of grade in her systems of groups, which teaches that 
all general principles of classification should be liberally inter- 
in others,—that we are compelled to allow each, for itself, to be 
iN a Sense its own interpreter. These facts make it the more 
dificult to give general principles that comprehensive form of 
éroups of this kind—and not those made by reference to some 
satisfactorily compared 
8p 2 
with one another in the determination of ordinal relations 
that while a type may run down into degraded forms, mere de- 
etadation is not a reason for breaking the group in two, an upper 
and a lower portion; and that types of very unequal rank may 
ont special study, will serve for a few illustrations. This class, 
although not ik of the most prominent in the Animal Kingdom 
alran 
and the mathematical law would not have been learned at all 
~:2,000,000,000 least 1,000 times greater than 
Sects; and aa gb exireneiane thus widely separate, the 
rhole Tange from one limit to the other is occupied by a num- 
mo! Species not exceeding one-fiftieth of those af ar 
a is Teason, therefore, for expecting a magnified display o 
8, 
