180 HEREDITY AND EVOLUTION IN PLANTS 



Again, as Taylor points out, most of the recent endem- 

 ics in the New York flora are not woody, the proportion 

 of woody species among the endemics (17 per cent.) 

 being essentially the same as for the entire flora (18.2 

 per cent.) Most of the endemics are probably accounted 

 for by generic and specific instability, that is, by the ten- 

 dency of existing forms to vary, at or near the edge of their 

 range, and for the variations to become established. At 

 least one is a case of "habitat" endemism; that is, the 

 endemic species is confined to a given locality because 

 suited to the environment afforded by that locality. This 

 is illustrated by Primus Gravesii, a saxitile form of the 

 beach-plum (P. maritima). 



Many factors are involved in the phenomena of en- 

 demism, and here, as in the case of discontinuous geo- 

 graphical distribution, each case must be carefully analyzed 

 by itself. In view of our present restricted knowl- 

 edge, we can generalize only with extreme caution. 



126. An Illustrative Study. As an illustration of the 

 application of evidence from various sources in an en- 

 deavor to decide the relative age of two large groups of 

 plants, herbs and woody plants (trees and shrubs), there 

 may be mentioned the recent work of Sinnott and Bailey, 1 

 who marshalled evidence from paleobotany, anatomy, 

 phylogeny, and phytogeography, as bearing on the rela- 

 tive antiquity of herbaceous and woody plants. Very 

 briefly summarized, their argument runs as follows: 



i. A study of fossil plants shows that the remains of 



1 Sinnott, Edmund W. and Irving W. Bailey. Investigations on the 

 phylogeny of the Angiosperms: No. 4. The origin and dispersal of 

 herbaceous Angiosperms. Ann. Bot. 112: 547-600. Oct. 1914. The 

 phraseology of the authors is freely incorporated in the above very brief 

 summary. 



