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."f These are the chief 

 and leading principles which Mr. MacLeay considers 



* Hor. Ent. 319. t Ibid. 518. 



