176 HEREDITY AND EVOLUTION IN PLANTS 



is should be noted that a considerable proportion of the 

 species of the strand vegetation of Hawaii are endemic, 

 but many of the introduced littorals are known to be 

 transported by ocean currents from the north Pacific. 1 , 



124. Distribution of Algae. And finally, to bring all the 

 great phyla under brief review, it may be mentioned that 

 facts of distribution of the Algae point to a great antiquity 

 for the group. This is not only in harmony with the 

 generally accepted evidence from comparative morphology, 

 but is substantiated by fossil remains, in early Paleozoic 

 rocks, of calcareous Siphonogamous forms related to liv- 

 ing calcareous forms. The absence of fossil remains of 

 non-calcareous green forms is readily explained by the 

 delicate nature of their tissues. 



125. Hypothesis of "Age and Area." As noted above 

 (p. 165), an endemic species is one found in a given local- 

 ity but not elsewhere. According to some botanists 2 

 endemism is a criterion of youth. The area occupied 

 by a species within a given country, argues Willis, varies 

 directly with its age within that country; that is, the longer 

 it has been a part of the flora, the wider the area it occupies, 

 so long as conditions remain constant. But Willis enumer- 

 ates various conditions that would interfere with the 

 operation of this law, including "chance" (i.e., causes not 

 understood), the action of man (clearing of forests, 3 etc.), 



1 Twenty-one littorals and eleven pseudo-littorals, out of a total of over 

 75, are listed as endemic by Vaughan MacCaughey. Bull. Torrey Bot. 

 Club, 45: 259-277. July, 1918. 



2 Willis, J. C. The relative age of endemic species and other controver- 

 sial points. Ann. Bot. 31:189-208. April, 1917. James Small (see p. 

 148) has characterized Willis's Age and Area hypothesis, as the most im- 

 portant contribution to geographical botany since the Origin of Species. 



3 Macrozamia Moorei is being systematically exterminated in Australia 

 because it is poisonous to cattle. 



