INTRODUCTION. 7 J 
as now practised on external characters alone, until he 
has thoroughly investigated each of them under all its 
different aspects. In order to do this, he who pro- 
poses additions to the list of our species should pos- 
sess, or have the opportunity of examining, complete 
series of specimens embracing those of every age, from 
the egg to maturity, and from every section of the 
country in which the}- occur. He should ascertain to 
what circumstances they are subjected in different lo- 
calities, and be able to mark the different modifica- 
tions which correspond to differences of temperature, 
soil, elevation, humidity, and to the kind and quantity 
of food. He should be acquainted with their retreats, 
and their habits, so far as these are likely to affect 
then- external characters. It is only after he shall have 
made a careful comparison of all the variations which 
he notices, that he will be able fully to understand the 
species, and to decide definitively and correctly upon 
its varieties. If he finds the characters of his proposed 
species melting gradually into those of one already well 
established ; or, in other words, if the interval between 
them is filled up step by step, by variations which, 
though differing but little from each other, serve to con- 
nect the extremes, he ought to consider the two to be 
specifically identical ; for if any other practice should be 
adopted, there would be absolutely no limit to the num- 
ber of species, and every department of zoology would 
consist only of the history of well-marked individuals. 
And besides all this labor, he should furthermore be 
