202 ON SYSTEMATIC ZOOLOGY. 
rough sketch of the leading peculiarities of the great 
divisions of animals, and the manner in which they are 
probably connected, than an accurate determination of 
the groups themselves, or a demonstration of their real 
affinities. More than this, perhaps, could not have been 
expected, considering the then state of science, and the 
herculean difficulties which the author had to surmount. 
The work in question has now become exceedingly 
scarce, and this will be an additional reason with us 
for communicating occasional extracts from it to the 
reader. Mr. MacLeay’s theory will be best understood 
by consulting his diagrams ; for he has not, as we have 
already remarked, defined any of the vertebrated groups. 
“Condensing, however, the result of his remarks, we 
shall state them as resolvable into the following proposi- 
tions: — 1. That the series of natural animals is con- 
tinuous, forming, as it were, a circle; so that, upon 
commencing at any one given point, and thence tracing 
all the modifications of structure, we shall be imper- 
ceptibly led, after passing through numerous forms, 
again to the point from which we started. 2. That 
no groups are natural which do not exhibit, or show an 
evident tendency to exhibit, such a circular series. 3. 
That the primary divisions of every large group are ten, 
five of which are composed of comparatively large 
circles, and five of smaller: these latter being termed 
osculant, and being intermediate between the former 
which they serve to connect. 4. That there is a tend- 
ency in such groups as are placed at the opposite points 
of a circle of affinity “ to meet each other.”* 5. That 
one of the five larger groups into which every natural 
circle is divided, ‘‘ bears a resemblance to all the rest, 
or, more strictly speaking, consists of types which repre- 
sent those of each of the four other groups, together 
. with a type peculiar to itself.’+ These are the chief 
and leading principles which Mr. MacLeay considers 
* Hor, Ent. 319. t Ibid. 518. 
