820 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 tiesof 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. 
