286 COMPARATIVE ANATOMY 



families related to one another in the first degree of cousin- 

 ship. A zoological family may be compared to groups of first 

 cousins related to one another in more distant degrees of 

 cousinship. An order, again, to groups of distant cousins 

 related by remoter ties of. tribal kinship. A class may be 

 compared to a number of tribes forming a nation, and a 

 phylum to a number of nations forming a great race, like the 

 Aryan or Mongolian race. The complete genealogy of a 

 tribe would show the descent of all the families composing 

 it from a common ancestor, the founder of that tribe, and 

 it would show also the various degrees of kinship not only 

 between the surviving families of the tribe but between all 

 the families existing at any given time in the past. Similarly, 

 the complete genealogy of a zoological order would show the 

 descent of all the species contained in it from a common 

 ancestor, and would show the degrees of kinship of all 

 existing species and of all pre-existing species at any given 

 period of time. But it need hardly be said that such a 

 complete genealogy does not exist and never will exist. There 

 is no continuous record. Zoologists can only infer the 

 degrees of affinity of species by their greater or less structural 

 resemblance to one another, aided by the very imperfect and 

 fragmentary evidence of descent furnished by extinct forms 

 whose remains have been preserved in a fossil state. Any 

 system of classification, therefore, depends, in the first place, 

 on the extent of our knowledge of the structural differences 

 which obtain among species, and in the second place, upon 

 the judgment shown by the framer of the system in grouping 

 together or separating species according to their greater or 

 less resemblances and differences. Consequently there is no 

 such thing as a final scheme of classification of any group. 

 Systems must change with the increase of our knowledge and 

 according to the varying judgments of the authors who frame 

 them. It also follows that there is more certainty about the 

 affinities of species and genera than of the higher divisions, 

 for it is easier to recognise near than remote relationships. 

 In fact, the limits assigned to classes, orders, and families are 

 somewhat arbitrary, and these divisions in different phyla of 

 the animal kingdom do not imply equal degrees of affinity. 

 Thus the classes in one phylum may stand in much closer 

 relationship to one another than classes in another phylum. 



