SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION 115 



according to the longer or shorter time it has been in existence. 

 An individual specimen is "typical" of a species, a species is 

 "typical" of a genus, etc., if it has not had time enough to be 

 modified away from the characters which such species or genus 

 expresses. Any set of individuals, that is, any progeny, which 

 become modified to a degree from their progenitors, introduce a new 

 type ; and continually increasing modification makes such a type 

 specific, generic, and so on, in succession of time. There must have 

 been a time, for example, when the Avian and Reptilian " types " 

 began to diverge from each other, or, rather, to branch apart from 

 their common ancestry. In the initial step of their divergence, 

 when their respective types were beginning to be formed, the differ- 

 ence may have been infinitesimal. A little farther along, the incre- 

 ment of difference became, let us say, equivalent to that which serves 

 to distinguish two species. Wider and wider divergence increased 

 the difference, till genera, families, orders, and finally the classes 

 of Beptilia 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 farthest removed from the reptilian type, — which is most 

 highly specialised by differentiation to the last degree from the char- 

 acters of its primitive ancestors. One of the Oscines, as a thrush or 

 sparrow, would answer to such a type, having lost the low, primi- 

 tive, generalised structure of its early progenitors, and acquired very 

 special characters of its own, representing the extreme modification 

 which the stock whence it sprang 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 farthest removed and the most modified derivatives of 

 such stock, the characters of which are consequently remodelled and 

 obscured to the last degree. Two opposite ideas have evidently 

 been confused in the use of the word " Type." They may be dis- 

 tinguished by inventing the word teleotype (Gr. reAeos, teleos, final, 

 i.e. accomplished or determined ; formed like teleology, etc.) in the 

 usual sense of the word type ; and using the word we already 

 possess, prototype (Gr. irpun-os, protos, first, leading, determining), in 

 the broader sense of the earlier plan whence any teleotype has been 

 derived by modification. Thus, Ichthyornis or Archmopteryx is proto- 

 typic of modern birds, any of which 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 

 Archmopteryx, so prototypic of modern birds, was a very highly 

 speciahsed 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 



