58 Mr.W.S. Mac eay on certain general Laws regulating 
In the first place, M. Fries lays it down as a rule, which is 
quoted above, that he admits no groups whatever to be natural 
unless they form circles more or less complete. Let us then 
apply this rule to what he terms his central group, and which 
he makes always to consist of two. Does this form a circle? If 
not, the group cannot be natural according to his own definition. 
If, on the other hand, its two component groups are each 
circles, then these are natural. ‘Thus the Ptilota will not form 
one circle, but two ; consequently they form two natural groups, 
which is furthermore proved by their parallel relations of ana- 
logy. If we turn to Fungi also, the Hymenini, according to 
M. Fries, do not form one circle, but two; one of Pileati, the 
other of Clavati ; so that instead of the Hymenomycetes forming 
four natural groups, viz. Sclerotiacei, Tremellini, Uterini, and 
Hymenini, they form, if our author be correct, five; viz. Scle- 
rotiacei, Tremellini*, Uterini, Pileati, and Clavati. 
But, to understand this still better, we had as well perhaps 
enter a little deeper into our author's theory. Every group, he 
says, which expresses well the character of the superior group 
to which it belongs, is called the centrum ; by this, not meaning 
the centre of a circle, but the site of the normal form or perfec- 
tion of the particular structure common to the superior group, of 
which it forms a part. The word perfection, even as here used, 
requires explanation ; for it does not, as might be supposed, in 
this place signify affinity to any particular group. Our author, 
on the contrary, most properly says, that the idea of perfection 
in structure has nothing to do with affinityt. “ Ipsa hzc affini- 
tas 
* This appears to be one of those interesting groups which connect the least per- 
fectly organized beings with those which are the most perfectly organized. In the de- 
partment of Hysterophyta it is to the Coniomycetes or lowest Fungi, what in the animal 
kingdom the Wermes are to the Acrita. 
+ To the general observations on this subject, as connected with the animal king- 
dom, 
