88 INTRODUCTION. 



tion restricted commonly within very narrow limits, and 

 in Avliicli intelligence is replaced by instinct. 



The impossibility of arranging animals in an unin- 

 terrupted Linnean 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 tliis purpose their affinities and 

 analogies have been studied, their external appearance 

 and nnnute 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 descenduig 

 series, have supposed that there are collateral lines, 

 more or less numerous, diverging from the maui 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 ; 



