260 PRINCIPLES OF ANIMAL BIOLOGY 



the common stock from which the species of a genus have sprung must 

 have existed at an earher time, in order that evolution could bring about 

 the degree of divergence now observed. In like manner, a family is made 

 up of genera which resemble one another more than they resemble other 

 genera, and their likeness is again a sign of affinity. But to account for 

 the greater difference between the extreme individuals belonging to a 

 family, evolution must have had more time; that is, the common source 

 of the members of a family must have antedated the common source of 

 the individuals of a genus. Orders, classes, and phyla are similarly re- 

 garded as having sprung from successively more remote ancestors, the 

 time differences being necessary to allow for the differences in the amount 

 of evolution. This statement is in general correct. However, since 

 evolution has probably not proceeded at the same rate at all periods, 

 nor in all branches of the animal kingdom at any one time, the time re- 

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

 Thus certain genera, in which evolution has been slow, are probably much 

 older than some families in which evolution has been rapid. 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, nevertheless, the groups of higher rank have sprung from ances- 

 try more remote than that of the groups of lower rank. 



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, the evidential value 

 of similarity in one or several structures unaccompanied by the similarity 

 of all parts is to be distrusted, since animals widely separated and dis- 

 similar 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; and there 

 are certain lizards (Amphisbsenidae) which closely resemble certain 

 snakes (Typhlopidse) in being blind, limbless, and having a short tail. 

 The early systematists were very liable to bring together in their classi- 

 fication analogous forms, that is, those which are functionally similar; or 

 animals which are only superficially similar. In contrast with the early 

 practice, the aim of taxonomists at the present time is to group forms 

 according to homology (see Chapter X), which is considered an indication 

 of actual relationship. Since a genetic classification must take into con- 

 sideration the entire animal, the search for affinities becomes an attempt 

 to evaluate the results of all morphological knowledge, and it is also 

 becoming evident that other things besides structure may throw light 



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