in Zoology and Botany. 185 
nervous system is superior in importance to the circulatory, 
and the latter superior to the digestive system, yet this subject 
is still in a very indeterminate state, and until our knowledge 
of physiology is much further advanced, disputes will always 
arise respecting the true position of certain species in the na- 
tural classification. Such differences of opinion, however, will 
continually diminish as our knowledge increases, and they are 
even now very few in comparison with the numerous facts in 
classification on which all naturalists are agreed. Much ma 
be effected by education and habit, which impart to the natu- 
ralist a peculiar faculty (termed by Linnzus a “ latent in- 
stinct”’) for appreciating the relative importance of physiolo- 
gical characters to the satisfaction of himself and others, even 
in cases where he is unable to explain the principles which 
determine his decision. 
Granting, then, that by combining the numéer of points in 
which any two species agree, with an estimate of the physio- 
logical importance of those several points of agreement, the 
naturalist may, in practice, form a tolerably exact conception 
of the degree of resemblance between them ; he will proceed in 
his construction of the natural system to place these species 
at greater or less distance from each other, in proportion to 
that degree of resemblance. If we suppose that by a repeti- 
tion of this process every species is placed in its true position, 
we obtain a definition of those much-disputed terms, affinity 
and analogy,—the former of which consists in those essential 
and important resemblances which determine the place of a 
species in the natural system, while the latter term (analogy) 
expresses those wnessential and (so to speak) accidental re- 
semblances which sometimes occur between distantly allied 
species without influencing their position in the system. 
With analogy, therefore, we have no further concern in the 
present discourse, as it is a principle in no way involved in 
the natural system. Affinity, on the contrary, forms the 
chief element in this inquiry; and to place species in the 
order of their affinities is to construct the natural system*. 
It appears from the above views that the natural system 
is an accumulation of facts which are to be arrived at only by 
a slow inductive process, similar to that by which a country 
is geographically surveyed. If this be true, it is evident how 
_ * T am aware that by many naturalists analogy is considered to be as im- 
portant an element in the natural svstem as affinity is. As the discussion 
of this question would lead us away from the present object, I will not enter 
upon it now, especially as my views respecting it are stated more at large 
in the Mag. of Nat. Hist. for May last, p..222 et seq. 
