PRINCIPLES OF TAXONOMY 251 



tions of the groups of high or low rank must not be too rigidly assigned. 

 Thus certain genera in which evolution has been slow are probablj^ 

 much older than some families in which evolution has been rapid. The 

 genus Lingula (a burrowing marine brachiopod found between tide lines) 

 has evolved very little. The modern animals differ only slightly from 

 fossil Lingula of Ordovician time, estimated by some to be 400,000,000 

 years old. This is an extreme instance of slow evolution: Lingula is 

 probably the oldest living genus. Many families, even orders, and some 

 classes must be younger than that. It is not improbable, also, that 

 some genera are quite as old as the families which include them; but in 

 no case can they be older. Furthermore, different groups are classified 

 by taxonomists of different temperaments, so that groups of a given 

 nominal rank may be much more inclusive (and hence older) in one 

 branch of the animal kingdom than in another. On the whole, neverthe- 



FiG. 206. — Analogous structures; legs of several animals. A, kangaroo; B, crayfish; C, 

 honeybee. {C from Met calf and Flint, "Destructive and Useful Insects.") 



less, the groups of higher rank have sprung from ancestry more remote 

 than that of the groups of lower rank. 



Judging Kinship. — The means of recognizing the kinship implied in 

 classification permit some differences of opinion. It is recognized that 

 likeness in structural characters is the chief clue to affinities. However, 

 similarity in one or several structures unaccompanied by the similarity of 

 all parts is to be distrusted, since animals widely separated and dissimilar 

 in most characters may have certain other features in common. Thus, 

 the coots, phalaropes, and grebes among birds have lobate feet but, as 

 indicated by other features, they are not closely related ; that is, the lobes 

 on their feet are analogous, meaning that they serve the same function. 

 Analogy is mdespread in the animal kingdom, since the same activities 

 must be carried on by animals of very different structure. Locomotion, 

 for example, is effected by legs of vei;y different kinds. The legs of a 

 kangaroo, a crayfish, and a honeybee (Fig. 206) are analogous, but their 

 structure is unlike. The skeleton is within the flesh in the first of these 

 but on the outside in the other two, and the materials of the skeleton 

 are different. The crayfish and the bee, though alike in the position of 

 the skeleton, differ in the number and character of the segments of the 

 leg. Another case of analogous structures is that of lungs and gills 



