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 ah- 

 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 a circle. 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. 



(39l.) 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 



