CLASSIFICATION. 13 



This leads to the following law : 



The successive degrees of development attained by the 

 animal kingdom depend upon the extent to which functional 

 labor is divided. 



II. 



CLASSIFICATION. 



CLASSIFICATION is the arrangement of objects into groups, 

 the limitations of which are determined by some precon- 

 ceived standard of comparison. 



Classifications are artificial or natural, as the standard 

 chosen is either false or true. Thus the classification of ani- 

 mals, according to Aristotle, is artificial, since the standard 

 assumed, viz., the coloration of the blood, is false. An arti- 

 ficial classification is the result of an imperfect knowledge of 

 the things classified. 



It is only by an intimate acquaintance with the anatomy, 

 physiology, and development of each animal, that we can 

 determine in what tissues, and at what time, the true stand- 

 ard for comparison exists. As a rule, the hard tissues, such 

 as the shell, bone, teeth, scales, etc., furnish excellent second- 

 ary characters; yet in sub-groups of Vertebrata, soft tissues, 

 as the blood and brain, afford reliable data. Sexual char- 

 acters are occasionally of value, as the mammary gland, in 

 Mammalia, and, in a less degree, the construction of the 

 penis in the same class. The male, in Vertebrata, conveys 

 the expression of potentiality of species; in lower types, it is 

 subordinate. 



Maturity very generally yields the true standard of com- 

 parison. Yet examples are not rare where it is best seen in 

 the young. The articulated condition of Lepas (barnacle) is. 

 noticed only in the larval stage of the animal. The chorda 

 dorsalis the distinctive feature of the Vertebrata disap- 

 pears (hi comparatively few instances only is it persistent^, 

 viz., Cartilaginous fishes) before birth. 



