VARYING IMPORTANCE OF CHARACTERS. 65 



of imperfections, but as the one which he regards as most free 

 from faults and difficulties, of any yet proposed. In almost 

 every instance, he has adopted the views of some eminent Natu- 

 ralist, who has paid especial attention to that particular group ; 

 and the authority whom he has chiefly followed, will be men- 

 tioned under each head. 



44. It is a question of the greatest importance in Classifica- 

 tion, what characters are to be relied on for distinguishing 

 species, what for separating genera, what for dividing families, 

 and so on. To this question no satisfactory answer can be 

 given ; not only because similar characters have very different 

 values in two distant classes ; but also because, even in groups 

 not remote from each other, they are of very different degrees of 

 importance. Thus in one genus, there may be a certain cha- 

 racter, such as colour, which is invariably the same in each 

 species ; so that they may be distinguished from each other by 

 it alone. On the other hand, there may be certain variations in 

 regard to form, which the Zoologist does not regard ; because 

 he knows that, in the group in question, there is much latitude 

 in this respect. But in another group, i\\e forms of particular 

 parts, even though they be of great minuteness, may be so 

 constant for each species, as to furnish very satisfactory charac- 

 ters for dividing them ; whilst the colours may be so liable to 

 variation, that no use whatever can be made of them in classifi- 

 cation. Some examples of this kind have been already given 

 ( 15, 16). Hence, no general rules can be laid down on this 

 very important subject ; and in each group of whatever rank, 

 whether it be a class, an order, a family, or a genus, it ought to 

 be ascertained what characters possess most fixity, and what are 

 to be put aside as unfit to be relied on. 



45. There are four principal types or plans, upon which the 

 members of the Animal kingdom are constructed ; and these 

 plans are found to exert a modifying influence over the conform- 

 ation of all the organs of the body. That which prevails in the 

 lowest group is the Radiated arrangement ; in which the mouth 

 is situated in the centre of the body, and the several organs 

 are disposed around it, so as to give to the whole structure a 



