154 HEREDITY AND EVOLUTION IN PLANTS 



coming established, at least in limited area and numbers, 

 even though the number of seeds introduced was compara- 

 tively small. Farmers in America can bear emphatic 

 but sad testimony to their practical helplessness to com- 

 bat successfully the spread through the hay fields of the 

 hated daisy or white-weed (Chrysanthemum leucanthemum} , 

 or the still more dreaded Devil's paint brush (Hieradum 

 aurantiacum). Both of these species are now common 

 weeds in America, though introduced from Europe, the 

 former almost, and the latter quite within the memory of 

 men now living. Wallace has stated that, if a million 

 seeds were brought to the British Isles by wind in one year, 

 there would be only ten seeds to a square mile. "The 

 observation of a life time might never detect one, yet a 

 hundredth part of this number would serve in a few cen- 

 turies to stock an island like Britain with a great variety 

 of continental plants." When we recall the enormous 

 mortality of seeds and seedlings, such facts as these enable 

 us to appreciate the importance to a species of an abundance 

 of spore and seed production, as, for example, in dandelions 

 and other composites, in ferns, and indeed in most plants. 

 117. Types of Distribution. There are two broad 

 types of geographical distribution; continuous, as in the 

 case of the bracken fern (Pteris acquilina); and discon- 

 tinuous, as in the case of the Osmunda family, where a 

 given species is found in widely separated localities, but 

 not in the intervening regions. Osmunda regalis (the 

 Royal Fern), for example, is known from eastern North 

 America, central and northern Asia, and Europe; Os- 

 munda Japonica from central and northern Asia and Japan 

 and the cinnamon fern (Osmunda cinnamomea) only from 

 eastern North America and Japan. The genus Dicrvilla, 



