INTRODUCTION 71 



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 Avho pro- 

 poses additions to the hst 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 

 comitry in which they occur. He should ascertain to 

 what circumstances they are subjected m 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 

 their external characters. It is only after he shall have 

 made a careful comparison of aU the variations which 

 he notices, that he will be able fully to vmderstand the 

 species, and to decide definitively and correctly upon 

 its varieties. If he finds the characters of his proposed 

 species melting gradually mto those of one already well 

 estabhshed ; 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 hmit 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 



