REQUISITES OF A NATURAL SYSTEM. 129 
it attempts to explain the analogies or resemblances 
between the individuals or divisions of one circular series, 
when they are compared with those of another series. 
It is evident that all natural objects possess two dif- 
ferent sorts of relationship: one which is immediate, 
and another which is remote. The goatsucker and the 
swallow exemplify the first of these relations. These 
genera are intimately connected by structure, habits, and 
economy ; both fly nearly in the same manner, and both 
live upon insects, captured in the same way: but the 
goatsucker, besides this relation, has evidently another to 
the bats, —by flying at the same hour of the day, and by 
feeding in the same manner. ‘The first relation is in- 
timate— the latter remote. Hence arises the necessity, 
imposed upon all who wish to develope the natural 
system, of possessing clear perceptions of these two sorts 
of relations ; and of becoming well acquainted with the 
difference between affinity and analogy.* The first is 
exemplified by the swallow and goatsucker ; the latter 
by the goatsucker and the bat. Now, as these varied 
relations or resemblances are so universal throughout 
nature, that they have been perceived since science first 
dawned upon man, it is obvious that a writer who makes 
no effort to explain them, or to draw a just distinction 
between such as are immediate and such as are remote, 
neglects one of the most striking and wonderful pecu- 
liarities of the natural system. Nor is a bare mention 
ef such relations the only notice which is required ; for 
that carries with it no results: the accuracy of his series 
must depend upon being able to prove that all these 
resemblances follow each other in a uniform pro- 
gression: because it has been repeatedly demonstrated 
that the contents of one circular group represent the con- 
tents of another circular group ; and this principle of the 
natural system has been now so much developed, that not 
a doubt can remain of its prevalence throughout nature. 
Anysystem, therefore, which aims at being natural, must 
* Preliminary Discourse on Nat. Hist. p. 182. 
K 
