322 PRACTICAL AND SCIENTIFIC ZOOLOGY. 
lar succession of affinities which is the first great principle 
of the natural system. Between the first group or genus 
of a natural series, and the last, there is no line of ab- 
solute distinction ; the one blends into the other ; and 
the series, to use the expressive phrase, “ returns into 
itself.” 
(390.) Let us now look to what are the natural 
divisions of acircle. Mr. Macleay considers that every 
circle first divides itself into five minor circles: hence 
the system which he has developed is called quinary; 
five being the primary number employed. As it is 
better that the student should understand this principle 
of division before he proceeds farther, we shall illus- 
trate it by calling his attention to the vertebrated 
animals, or such as have an internal bony skeleton ; the five 
classes of which follow each other in the series mentioned 
in the last paragraph. We begin with birds and qua~ 
drupeds, proceed to fishes and amphibious animals, and 
end with reptiles. Here, then, are five divisions of ver- 
tebrated animals, each of which, upon investigation, are 
found to constitute a separate circle of their own; these, 
again, contain five minor circles. Let any one of these 
minor circles be selected for more close examination, and 
the same principle is discovered: every circle, in short, 
whether large or small, comprehensive or limited, con- 
tains five minor circles, and thus we go on, gradually 
descending from the highest to the lowest divisions, 
until we come to the smallest groups in nature, formed 
by the first assemblage of individual species. 
(391.) According to this theory, we have five prin- 
cipal divisions in every circle, the nature of which must 
now be explained. Two of these divisions are called 
typical ; because they contain those animals which! ex- 
hibit the greatest perfection of those particular qualities 
which more or less belong to all the five. Thus, the 
vertebrated and the annulose animals are the typical 
groups of the animal kingdom. The birds of prey and 
the perching birds are the typical orders of the feathered 
tribes. The apterous and the winged insects are typical 
