On the present state ofZooloyy. 23 
as are connected with no peculiarity whatever of habits or economy, 
in short, so little attempt made to ascertain the limits of variation in 
the case of those characters to which recourse is had for the purpose 
of systematic division. There is one observation in particular we 
would wish to make, the truth of which must be apparent to all who 
have paid the least attention to the structure of animals ; and that is, 
that organs of the same kind, and performing exactly the same 
functions, do not necessarily afford characters of the same value in 
different groups. This circumstance has been much overlooked. We 
not unfrequently see naturalists taking as the basis of their arrange- 
ment a system of organs, which, in certain classes, are universally 
allowed to be of the first importance, but which, in the group under 
their consideration, are subject to such modifications, as clearly indi- 
cate the subordinate influence which they exert over the economy. 
We might point out several instances in which this error shows it- 
'self. We shall, however, content ourselves with referring to the 
pulmonary and trachean Arachnida, which have been made distinct 
classes by some naturalists, on the ground of differences in their res- 
piratory organs, which they would seem to think entitled to as much 
regard in the case of the Aunulose animals, as all allow them to de- 
serve in that of the Vertebrate. 
The subject of the value of characters is closely connected with a 
question of great importance,- and which, in the present state of the 
science, calls for particular investigation. What are the limits of 
variation assigned to species 9 We conceive this to be a problem, to 
which, in these days, we shall do well in directing the attention of 
naturalists. We need hardly say how general has been the complaint 
of late years, that species are now multiplied to an excess, quite over- 
whelming to those who wish to enter into the details of the science, 
and bidding fair in time to exhaust all the resources of a pure and 
correct nomenclature. It is undoubtedly true, that to distinguish and 
point out even the slightest variation of character in animals is of 
importance, not merely as helping others to identify those which have 
been the subject of our observation, but as a necessary step to a 
knowledge of their true affinities. We are, however, only half com- 
pleting this matter, if we stop there, without proceeding to enquire 
whether these variations are due to a specific difference, or whether 
they may have resulted from the action of external or other causes. 
The evil above alluded to has taken deep root enough in our own 
country ; but in Germany it would seem to have attained to a yet 
greater pitch. M. Brehm, an ornithologist well known for the ex- 
tent to which he has multiplied the birds of Europe, a few years back 
