188 CRITIQUES AND ADDRESSES. [ix. 



the ^oplotheridw, and the Ruminants. The Anoplo- 

 tlieridw are intermediate between the first and the last ; 

 but this does not tell us whether the Ruminants have 

 conie from the Pigs, or the Pigs from Ruminants, or both 

 from Anoplotheridce, or whether Pigs, Ruminants, and 

 AnoplotheridcB alike may not have diverged from 

 some common stock. 



But if it can be shown that A, B, and C exhibit suc- 

 cessive stages in the degree of modification, or speciali- 

 zation, of the same type ; and if, further, it can be proved 

 that they occur in successively newer deposits, A being 

 in the oldest and C in the newest, then the intermediate 

 character of B has quite another importance, and I should 

 accept it, without hesitation, as a link in the genealogy 

 of C. I should consider the burden of proof to be 

 thrown upon anyone who denied C to have been derived 

 from A by way of B, or in some closely analogous fashion ; 

 for it is always probable that one may not hit upon the 

 exact line of filiation, and, in dealing with fossils, may 

 mistake uncles and nephews for fathers and sons. 



I think it necessary to distinguish between the former 

 and the latter classes of intermediate forms, as intercalary 

 types and linear types. "When I apply the former term, 

 I merely mean to say that as a matter of fact, the form 

 B, so named, is intermediate between the others, in the 

 sense in which the Anoplotherium is intermediate between 

 the Pigs and the Ruminants without either affirming, 

 or denying, any direct genetic relation between the three 

 forms involved. When I apply the latter term, on the 

 other hand, I mean to express the opinion that the forms 

 A, B, and C constitute a line of descent, and that B is 

 thus part of the lineage of C. 



From the time when Cuvier's wonderful researches 

 upon the extinct Mammals of the Paris gypsum first made 

 intercalary types known, and caused them to be recognized 



