100 D. E. SAVAGE 



fication. Mayr (1946) uses this criterion extensively; his example (p. 13) 

 of the lark family is illustrative: "The larks are a family of more than 70 

 species and are represented in all parts of the Old World. Only certain 

 subspecies of a single species occur in the New World. There can he no 

 shadow of doubt concerning the family's Old World origin" [italics are 

 mine]. Perhaps not in Mayr's example but in many similar statements, 

 such an approach may be very tenuous and often misleading unless bio- 

 stratigraphic, sedimentologic, mineralogic, and geochemical studies indi- 

 cate that the proposed center of origin was comparable in ecologic diver- 

 sity with the inferred non-origination areas. 



The interpreted center of dispersal, as evidenced by the distribution 

 and case history of living forms, may be altogether different from the 

 center of origin, particularly in the bradytelic phyla. The environment of 

 origin for many groups of organisms has shifted across great distances 

 since their inception. See, for example, Chaney (1936, 1940), Axelrod (1952 

 and earlier papers), and Stebbins (1950, Chap. XIV). 



A complication may arise also in the consideration of animal groups 

 that become less diversified in the later episodes of their phyletic history 

 for reasons that may not be apparent in the sedimentary record. The liv- 

 ing Didelphoidea (opossums) are most diversified and include possibly 

 the most advanced members in the Neotropical region, but all living 

 didelphoids show less morphological variation than the late Cretaceous 

 Nearctic forms that are presently considered members of the group^ Thus 

 we conclude that the Neotropics are an asylum of diversified didelphoids 

 and that these Neotropical forms had their ecologic counterparts in late 

 Cretaceous Nearctica. Moreover, we must look for the origin of the 

 didelphoids in sediments that are lower in the stratigraphic column than 

 the presently known fossil sites. 



Finally, the third criterion is the more revealing for the smaller taxa. 

 Later diversification within an order, for example, may have little geo- 

 graphic relationship to the origin of the first and most primitive species 

 of the order. 



4. Phyletic age of the group is important. Phyletically ancient forms, 

 groups that have changed relatively little (in hard parts, of course) 

 through a long interval of time, are frequently known from restricted 

 or from scattered, relict type occurrences. Pertinent examples may be 

 found in many living amphibians, reptiles, and invertebrates, and even 

 in some mammals. Living lemurids in Madagascar have close relatives 

 in the early Cenozoic deposits of North America and Europe. The South 

 American, Asiatic and African distribution of living hystricomorph ro- 

 dents may represent an unrecorded Paleocene transworld dispersal of a 

 primitive stock, as suggested by Landry (1957). There are many records 



1 Only jaws and dentitions are known for the late Cretaceous forms, but greater size 

 range and greater diversity of tooth structure are clearly indicated. Paleomammalogists 

 infer, therefore, more diversified gross morphologies, diets, and habits in these early 

 types. 



