258 WALKS AND TALKS. 



XL/V. ANTICIPATION AND RETROSPECT IN 



COMPREHENSIVE TYPES. 



WHAT I wish now to set forth is a principle of very pro- 

 found significance. It is a general truth in the nature of the 

 succession of organic types; and I will endeavor to make it 

 plain by citing some of the striking illustrations of it. 

 From the working of this principle, it results that the crea- 

 tures of any age often unite in themselves some characters of 

 a group actually existing, with characters of a group not yet 

 in existence. This is anticipatwn. Such a union forms a 

 "prophetic type," as Agassiz used to express it. It is also a 

 comprehensive type. Also, from the working of this principle, 

 it results that the creatures of any age often unite in them- 

 selves some characters of a group actually existing, with 

 characters of a group which was dominant in a former age 

 whether still existing or not. Such a union forms a retro- 

 spective type. This is also comprehensive. But in some com- 

 prehensive types we find a union of characters none of 

 which belong to any fairly circumscribed existing group. 

 They are all prophetic or anticipatory of groups which are 

 destined to be defined in the future. 



Take first, the early Ganoids for a good example. Their 

 vertebrae were generally concavo-convex. This is a reptilian 

 character. Nearly all reptiles, living and extinct, possess 

 such vertebrae, while all typical or proper fishes possess bi- 

 concave vertebrae. Now the early Ganoids were not reptiles, 

 and had no claim to concavo-convex vertebrae. There had 

 never been a reptile in existence when these Ganoids first 

 lived when 0-nych! -o-dus of Ohio, for instance, flourished. 

 If we may attribute to the ancient Ganoids, certain other 

 characters which belong to modern Ganoids, like the gar- 

 pike, we should say they possessed an opening or glottis in 

 the back part of the mouth, and that a passage existed from 

 this to the air bladder ; and that the latter organ was coarsely 



