98 INTRODUCTION 
construct such a series has been a failure. Of late 
years, 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 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 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 are 
put forth, each as the true plan or system 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 par- 
allel to it, and, after an interval of greater or less extent, 
merging again with it. 
