56 FORREST SHREVE 



difficult to believe that any present physical factors are restrict- 

 ing them. The history of these plants has been one of extinction 

 and retraction, due to changes in their old extended environment. 

 They are now unable to regain their old ranges, or even to 

 spread over relatively short distances. These species may well 

 be looked upon as physiologically and genetically decadent. 

 They are probably limited in distribution because they have 

 failed to change their requirements to that slight extent which 

 might permit a greater extension under altered conditions, and 

 they are old simply because they have failed to change to that 

 greater extent that would have reacted upon form in such a 

 manner as to make us regard the resulting plants as a distinct 

 species from the old ones. 



In addition to the old and restricted, or relict, species we have 

 another class of plants of limited range, those which are appar- 

 ently just appearing on the scene and have not yet had time to 

 occupy the entire area in which they might be expected to find 

 congenial conditions. These plants are generally members of 

 large genera such as Opuntia, Antennaria and Crataegus, of else 

 of genera which have been recently subdivided, whereas the relict 

 species are usually members of small genera. It is not always 

 easy to decide whether a given restricted distribution is of the 

 relict type or of the '^novitiate" type, as we may call the emerg- 

 ing species. Our decision — in the lack of other evidence — is 

 apt to be based upon the existence or absence of fossil records 

 of the species or its allies, or else upon the size and phylogenetic 

 position of the genus or family. Such a restricted conifer as 

 Pinus inayriana may well belong in either class, and the mem- 

 bers of many small or monotypic genera of Mexican Compositae 

 may also stand in a doubtful position. 



A very considerable number of our plants belong neither to 

 the relict nor to the novitiate class. They are of such age that 

 they have had time to extent themselves as far as the subtle 

 fa^ctors of their environment, the crude power of great barriers, 

 or the operation of suitable agents of dispersal have permitted 

 them, and their course has not run so far that cHmatic and 

 orographic calamities have overtaken and restricted them. 



