98 INTRODUCTION. 



construct such a series has been a faikire. Of late 

 years, the attention of naturahsts 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 prin- 

 ciple of natural arrangement. For this purpose their 

 affinities and analogies have been studied ; their exter- 

 nal appearances and their minute internal anatomy have 

 been examined ; and the whole economy of their hves has 

 been sought out. A comparison of these, aided by 

 acute observation and ingenious reasoning, has resulted 

 in the promulgation of several hypotheses, which are 

 put forth, each as the true plan or system followed 

 by nature in the creation of hving 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 par- 

 allel to it, and, after an interval of greater or less extent, 

 merging again with it. 



