FAMILY CHARACTERISTICS. 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,— tliat 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 toform. The Dog is comparatively slender, 
with legs adapted for running and hunting his 
prey; the Bear is heavier, with shorter limbs; 
ef 
