FAMILY CHARACTEEISTICS. 113 



must be based, like all the other divisions, upon 

 some combination of structural characters pecu- 

 liar to them. We have seen that Branches are 

 founded upon the general plan of structure, 

 Classes upon the execution of the plan, Orders 

 upon the greater or less complication of a given 

 mode of execution, and we shall find that form, 

 as determined by structure, characterizes Fami- 

 lies. I would call attention to this qualification 

 of my definition ; since, of course, when speak- 

 ing of form in this connection, I do not mean those 

 superficial resemblances in external features al- 

 ready alluded to in my remarks upon Parallel or 

 Collateral Types. I speak now of form as con- 

 trolled by structural elements ; and unless we 

 analyze Families in this way, the mere distin- 

 guishing and naming them does not advance our 

 science at all. 



Compare, for instance, the Dogs, the Seals, 

 and the Bears. These are all members of one 

 order, that of the Carnivorous Mammalia. 

 Their dentition is peculiar and alike in all (cut- 

 ting teeth, canine teeth, and grinders), adapted 

 for tearing and chewing their food ; and their in- 

 ternal structure bears a definite relation to their 

 dentition. But look at these animals with refer- 

 ence to form. The Dog is comparatively slender, 

 with legs adapted for running and hunting his 

 prey ; the Bear is heavier, with shorter limbs ; 



H 



