320 PRACTICAL AND SCIENTIFIC ZOOLOGY. 



appear,, as it were, to wander out of the series, as if they 

 were joined to others which neither preceded nor followed 

 them directly. As we advance, these deviations out of 

 the simple and direct line, which we are vainly endea- 

 vouring to trace, become more frequent and more per- 

 plexing ; we relinquish the investigation as hopeless, 

 and become convinced that, however rational or probable 

 such a supposition may appear in theory, it is utterly 

 impossible to exemplify it in its details. This has been 

 the conclusion, avowed or implied, at which every phi- 

 losopher, who has investigated the subject, has ultimately 

 arrived. 



(387.) By another theory, the system of nature has 

 been compared to an extensive piece of network, where 

 the different knots and meshes represent the mode in 

 which different animals are connected together in every 

 direction. The advocates of this theory, however, con- 

 sider it hopeless to discover by what general principle 

 these innumerable ties of affinity are affected, and content 

 themselves with asserting the probability of the theory, 

 without venturing to its demonstration. It was not 

 until the beginning of the present century that a new 

 light was thrown upon this interesting question. A be- 

 lief that there existed a branched or double progression 

 of affinities, which ultimately met together, and united, 

 originated with Lamarck ; but this was only the " sha- 

 dowing forth" of the discovery that affinities are really 

 circular, demonstrated by Messrs. MacLeay and Fries. 

 This is called the Circular Theory ; and has now been 

 so extensively verified, that it may be pronounced the 

 first law of natural arrangement. As the details of 

 this theory have been already enlarged upon, our pre- 

 sent object is merely to explain its leading peculiarities 

 by the most simple and familiar illustrations. 



(388.) The circular series, by which the gradations 

 of natural beings are regulated, may be thus explained. 

 We will suppose it proved that, in the scale of nature, 

 birds are followed by quadrupeds, quadrupeds by fishes, 

 fishes by frogs, and these latter by reptiles and tortoises. 



