PRINCIPLES AND PRACTICE OF CLASSIFICATION. 75 



tive types were beginning to be fornied, the difference must have been infinitesimal. A little 

 further along, the increment of difi'erence became, let us say, equivalent to that which serves 

 to distinguish two species. Wider and wider divergence increased the difi'erence till genera, 

 families, orders, and finally the classes of Reptilia and Aves, became established. In one sense, 

 therefore, — and it is the usual sense of the term, — the " type " of a bird is that one which is 

 furthest removed from the reptilian type, — which is most highly specialized by diflereutiation 

 to the last degree from the characters of its primitive ancestors. One of the Oscines, as a 

 Thrush or Sparrow, would answer to such a type, having lost the low, primitive, generalized 

 structure of its early progenitors, and acquired very special characters of its own, representing 

 the extreme modification which the stock whence it sprung has undergone. In a broader 

 sense, however, the type of a bird is simply the stock from which it originated ; and in such 

 sense the highest birds are the least typical, being the furthest removed and the most modified 

 derivatives of such stock, the characters of which are consequently remodeled and obscured 

 to the last degree. Two opposite ideas have evidently been confused in the use of the word 

 " Type." They may be distinguished by inventing the word teleotype (Gr. Ti\eosi teleos, final, 

 i. e., accomplished or determined) in the usual sense of the word type, and using the word we 

 already possess, prototype (Gr. Trpayros, protos, first, leading, determining), in the broader sense 

 of the earlier plan whence any teleotype has been derived by modification. Thus, Archceop- 

 teryx is prototypic of modern birds, all of which latter are teleotypic of their ancestors. It 

 may be further observed that any form which is teleotypic in its own group, is prototypic of 

 those derived from it. Thus, the Archceopteryx, so prototypic of modern birds, was a very 

 highly specialized teleotype of its own ancestry. A little reflection will also make it clear that 

 the same principle of antitypes (opposed types) is applicable to any of our groups in zoology. 

 Any group is teleotypic of the next greater group of which it is a member; prototypic of the next 

 lesser one. Any species is teleotypic of its genus; any genus, of its family; any family, of its 

 order ; and conversely ; that is to say, any species represents one of the ulterior modifications 

 of the plan of its genus. The Class of Birds, for example, is one of the several teleotypes of 

 Vertebrata, i. e., of the vertebrate plan of structure; representing, as it does, one of several 

 ways in which the vertebrate prototype is accomplished. Conversely, the Class of Birds is 

 prototypical of its several orders, representing the plan which these orders severally unfold in 

 different ways. And so on, throughout any series of animals, backwards and forwards in the 

 process of their evolution; any given form being teleotypic of its predecessors, prototypic of its 

 successors. All existing forms are necessarily teleotypic — only prototypic for the future. Pro- 

 totype, in the sense here conveyed, indicates what is often expressed by the word archetype. 

 But the latter, as I understand its use by Owen and others, signifies an ideal plan never actually 

 realized; the "archetype of the vertebrate skeleton," for example, beiug something no verte- 

 brate ever possessed, but a theoretical model — a generalization from all known skeletons. The 

 correspondence of my use of "prototypic" with a common employ of "archetypic," and of 

 " teleotypic " as including both " attypic " and " etypic," is noted below.^ 



The actual and visible genetic relationships of living forms being practically restricted to 

 individuals of the same species — parents and offspring " specifically "' identical — it would seem 

 at first sight that species must be the modified descendants of their respective genera, in order 



* '^Archetypical characters are those which a group derives from its progenitor, and with which it commences, 

 but which in much modified descendants are lost ; such, for example, is the dental formula of the Educabilia (M ^ PM } 

 C n 3 X 2), — a formula, as shown by Owen, very prevalent among early members of the group, but generally departed 

 from more or less in those of the existing faunas. Alli/pical characters are those to the acquisition of which, as a matter 

 of fact, we find that forms, in their journey to a specialized condition, tend . . . Etypicnl characters are exceptional 

 ones, and which are exhibited by an eccentric offshoot from the common stock of a group." {GUI, Pr. Am. Assoc. Adv. 

 Sci. XX, 1873, p. 293.) To illustrate in birds: A generalized lizard-like type of sternum \s archetypic oi any bird's ster- 

 num. The sternum of the lizard-like animals whence birds actually descended is prototypic ; the keeled sternum of a 

 cariuate bird is ntlypicnl in most birds, etypical in the peculiar state in which it is found in Stringops ; but equally 

 teleotypic in both instances. 



