Families, as they exist in Nature, are based upon peculiarities of form as related to structure; but though a very large number of them have been named and recorded, very few are characterized with anything like scientific accuracy. It has been a very simple matter to establish such groups according to the superficial method that has been pursued, for the fact that they are determined by external outline renders the recognition of them easy and in many instances almost instinctive; but it is very difficult to characterize them, or, in other words, to trace the connection between form and structure. Indeed, many naturalists do not admit that Families are based upon form; and it was in trying to account for the facility with which they detect these groups, while they find it so difficult to characterize them, that I perceived that they are always associated with peculiarities of form. Naturalists have established Families simply by bringing together a number of animals resembling each other more or less closely, and, taking usually the name of the Genus to which the best known among them belongs, they have given it a patronymic termination to designate the Family, and allowed the matter to rest there, sometimes without even attempting any description corresponding to those by which Genus and Species are commonly defined. For instance, from _Canis_, the Dog, _Canidae_ has been formed, to designate the whole Family of Dogs, Wolves, Foxes, etc. Nothing can be more superficial than such a mode of classification; and if these groups actually exist in Nature, they must be based, like all the other divisions, upon some combination of structural characters peculiar to them. We have seen that Branches are founded upon the general plan of structure, Classes on the mode of executing 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 Families. I would call attention to this qualification of my definition; since, of course, when speaking of form in this connection, I do not mean those superficial resemblances in external features already alluded to in my remarks upon Parallel or Collateral Types. I speak now of form as controlled by structural elements; and unless we analyze Families in this way, the mere distinguishing 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, (cutting teeth, canine teeth, and grinders,) adapted for tearing and chewing their food; and their internal structure bears a definite relation to their dentition. But look at these animals with reference to form. The Dog is comparatively slender, with legs adapted for running and hunting his prey; the Bear is heavier, with shorter limbs; while the Seal has a continuous uniform outline adapted for swimming.