FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 103 



[italics are mine], and on the misconception that north temperate to 

 tropical climatic zones have not shifted much during the Cenozoic. 

 It has been effectively demonstrated that there was a tremendously 

 greater latitudinal expanse of tropical, subtropical, and warm- 

 temperate climates during the late Mesozoic and early Cenozoic 

 interval when many of the now surviving orders and families and 

 some of the genera of animals were actually beginning; even though 

 the orientation of climatic belts was similar to the present. 



Holarctica, although defined originally by the fauna living in the 

 present temperate and frigid climates, means much more than the 

 extant north temperate frigid region. When visualized through the 

 span of its geochronologic age, most of Holarctica was tropical and 

 subtropical throughout the first two-thirds of mammalian history. 

 Perhaps the inception date for Holarctica should be specified as the 

 time when oceanic and continental segments of the earth's crust 

 were first arranged roughly as at present, whenever that might 

 have been. I can see no utility for the term prior to the time of 

 abundant land life, however, and probably not before the "Age" of 

 endotherms. To the paleomammalogist, Holarctic is a shorthand 

 term to signify: (1) the fauna of the northern world continent; 

 (2) the northern world continent, characterized by its fauna through 

 the geochronologic age of the fauna; or just (3) the world continent 

 of North Africa, Eurasia, and North America through a geochrono- 

 logic interval. The term is therefore, a generalization, involving 

 geography, organisms, and time duration, with varying emphasis on 

 these respective constituents. Holarctica, like fauna, flora, strati- 

 graphic zone, species, and many other terms in our technical dic- 

 tionaries is entrenched by usage and is not vitiated because it has 

 been used with differing connotations or because certain organisms 

 do not have a dispersal history that can be described most effectively 

 by using Holarctic and sister terms. 



Many groups of mammals may have originated in tropical, sub- 

 tropical, or warm-temperate biotopes, whether in northern latitudes 

 or in the present "tropics." Entire orders of mammals may be auto- 

 chthonous and endemic to the warmer and more humid areas, prob- 

 ably for varied reasons. As examples we may take the Primates, 

 the Dermoptera, and the Chiroptera (with most exceptions). I sug- 

 gest to the other contributors in this symposium that, if a "tropical" 

 origin is proposed for a group of animals, they carefully specify 



