FLORA OF THE SANTA BARBARA ISLANDS. 221 



and Santa Cruz but even extremely upon the island itself. 

 These local variations upon the islands are caused mainly 

 by more or less exposure to the strong and almost incessant 

 northerly winds. A fine example of this class of variation 

 is afforded by the Dendvomecon of Santa Rosa, which, ac- 

 cording to its location, assumes the mainland form with 

 narrow, rough-edged, or the insular form of Santa Cruz, 

 with broad, smooth-margined leaves. 



The variation of a plant of one island from its form upon 

 another island is always interesting, often puzzling, and 

 from the lack of connecting forms is, in many instances, 

 considered specific. The two species of the insular genus 

 Lyonothamnus , one from the island of Santa Catalina, and 

 the other from Santa Cruz and Santa Rosa, are so closely 

 related, that, as far as known, their specific distinctness 

 rests only upon the form of the leaf, and the closeness of 

 their relationship is still further strengthened by the fern- 

 like divisions of L. asplenifolius appearing upon many of 

 the leaves of L. florihundus. 



The two species of Lyonothainims will probably be con- 

 sidered distinct, and the great extent of water separating 

 the islands renders the existence of connecting forms doubt- 

 ful. 



But Ceanothus arhoreus of Santa Cruz and its representa- 

 tive upon Santa Rosa, are examples of different forms of dif- 

 ferent islands that can hardly be considered specifically 

 distinct, although the specimens are apparently as much 

 so as those of many received species of this senus. 



Between these examples of extreme variation and the 

 larger number of plants which retain upon the different 

 islands a similar form, numerous instances of more or less 

 deviation might be given. 



Insular variation, or the variation of the island species 

 from their mainland representatives, is more complicated, 

 but seems mainly either to be the result of an insular cli- 

 mate or a separation of their respective habitats for a long 



