88 LSTRODUCTION. 
tion restricted commonly within very narrow limits, and 
in which intelligence is replaced by instinct. 
The impossibility of arranging animals in an unin- 
terrupted Limiean series being now acknowledged, and 
every practical attempt to construct such an one having 
failed, the attention of naturalists has been turned in 
another direction, and much time has been given to the 
investigation of the structural and functional relations 
of the several groups and species to each other, with a 
view of deducing from them the true principle of natu- 
ral arrangement. For this purpose their affinities and 
analogies have been studied, their external appearance 
and minute internal anatomy have been examined, and 
the whole economy of their lives has been sought out. 
A comparison of these, aided by acute observation and 
ingenious reasoning, has resulted in the promulgation of 
several hypotheses which have been put forth, each as 
illustrating the plan followed by nature in the creation 
of living beings, and which should therefore be adopted 
as the basis of zoological classification. The authors of 
some of these, though admitting a generally descending 
series, have supposed that there are collateral lines, 
more or less numerous, diverging from the main series, 
but continuing parallel to, and after an interval of 
greater or less extent, merging again with it. 
Others, and the most numerous class, have conceived 
that, at whatever point we commence, we shall, by 
tracing the gradations of organization and the connect- 
ing affinities of groups, arrive at the same pomt again ; 
