ORGANIC EVOLUTION 63 



DISTRIBUTION OF VEGETABLE FORMS. Especially is 

 the distribution of seeds of plants accomplished by 

 water, wind, and birds. It is hardly necessary to dis- 

 cuss further such wide distribution in the light of its 

 being accessory in producing many new variations, in 

 widely distributed areas. Wherever seeds are thus car- 

 ried, and dropped in soil, and climate, not sufficiently 

 different from those from which the seeds came, to pre- 

 vent fructification, there would arise some variation 

 from the parent stock, due to environment, and often 

 a great deal of variation. Experiment has shown that 

 many seeds are not injured by passing through the diges- 

 tive system of birds. Locusts carry tiny seeds of grasses 

 from mainland to island, and from island to mainland. 



THE GLACIAL EPOCH. The glacial epoch distributed 

 the arctic flora as far south as the ice extended, in both 

 continents. That flora still remains on the tops and 

 sides of mountains, and corresponds today with the same 

 species of high latitudes, in proportion to the altitude 

 of the mountains. For example, that on top of the 

 White Mountains of New England is the same as that of 

 Labrador ; and that of the Rocky Mountains of the real 

 arctic regions. The lowlands both in Europe and 

 America, as the glaciers receded, not being congenial to 

 the seed left by the melted ice, failed to produce the 

 arctic flora, but resumed that which was adapted to such 

 climate. The identity of plants, on mountain tops in 

 Europe and America, is thus accounted for in a natu- 

 ral way. 



The importance of glaciers as geographical distribu- 

 tors of the florae and faunae of the earth is shown the 

 reader by Mr. Croll's theory of the alternate and 

 rhythmical occurrences of glacial epochs, in the north 

 and south hemispheres. The period he claims is about 



