VAEYING IMPORTANCE OF CHARACTERS. 63 



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 species 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, the forms of particular 

 parts, even though these 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 five 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 con- 

 formation of all the organs of the body. The type of the lowest 

 of these groups is generally considered to be that of the simple 

 nucleated cell (see ANIM. PHYS.), which performs all the func- 

 tions of animal life, although we can find no trace of special 

 organs for the execution of most of them. These animals, to 

 which the name of PROTOZOA is given in allusion to the sim- 

 plicity of their structure, are those which approach most closely to 

 the plants, in fact between them and the lowest plants it is often 

 difficult to draw a line of demarcation. The type which prevails in 

 the second 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 



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