350 PALAEONTOLOGY AND EVOLUTION xi 



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 inter- 

 mediate 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 re- 

 searches upon the extinct Mammals of the Paris 

 gypsum first made intercalary types known, and 

 caused them to be recognised as such, the number 

 of such forms has steadily increased among the 

 higher Mammalia. Not only do we now know 

 numerous intercalary forms of Ungulata, but M. 

 Gaudry's great monograph upon the fossils of 

 Pikermi (which strikes me as one of the most 

 perfect pieces of palseontological work I have seen 

 for a long time) shows us, among the Primates, 

 Mesopithecus as an intercalary form between the 

 SemnopitJieci and the Macaci ; and among the 

 Camivora, Hycenictis and Ictitherium as intercalary, 



