156 DYNAMICAL GEOLOGY. 



7. Calcareous Algae, called Niillipores (page 147), serve to protect grow- 

 ing Corals and the margins of coral reefs from wear. Ordinary seaweeds 

 often cover and protect the rocks of a coast nearly to high-tide level ; in the 

 higher latitudes the Fucoids (as Macrocystis pyrifera) are sometimes 200 to 

 300 yards long, and the broad green belt off a coast breaks the force of 

 incoming waves so that the rocks are saved from their destructive action. 



The common earthworm, as Darwin has shown (1881), transfers a great 

 amount of earth or soil in the pellets it discharges at the surface. He found 

 that the weight so transferred per acre in a year in four cases was 7 -56, 14-58, 

 164, and 18-12 tons. Lobworms, of seashores, are even greater workers, 

 according to C. Davison, who reports that the amount of sand carried up each 

 year on the shores of Holy Island, Northumberland, was equivalent to 1911 

 tons per acre (1891). Marmots {Spermatoj^hilus Eversmani), in the Caspian 

 steppes, bring great quantities of earth to the surface. In a few years after 

 their introduction they had brought up 75,000 cubic meters of earth to the 

 square mile. (Muschketoff, 1887.) 



Transpoeting Effects. 



1. Seeds caught in the feathers, hair, or fur of animals, or contained in 

 the mud adhering to their feet, are transported from place to place. 



2. Seeds are eaten by animals as food, or in connection with their food, 

 and are dropped in another region undigested. At the Solomon Islands, fruit- 

 pigeons carry fruit and seeds in their crops, and have thus planted the land 

 with trees from other islands. (Guppy.) 



3. Ova of fish, reptiles, and inferior animals are supposed to be transferred 

 from one region to another by birds and other animals. Authenticated 

 instances of this are wanting. 



4. Floating logs and seaweeds carry Mollusks, Crustaceans, Worms, and 

 other species from one region to another, over the broadest oceans, along the 

 courses of marine currents. In tropical countries, islands of shrubbery and 

 trees often float away from estuaries into the sea, bearing with them land, 

 fresh-water shells, and other terrestrial species, which there become mingled 

 with marine shells. A Boa constrictor once floated, on the trunk of a cedar, 

 from Trinidad off South America to the island of St. Vincent — a distance of 

 at least 200 miles. The great floating seaweed areas of the Sargasso Sea in 

 the Atlantic are the dwelling-place of vast numbers of marine species, includ- 

 ing Fishes, Mollusks, Crustaceans, Worms, etc. 



5. Migrating tribes of men carry, in their grain, or otherwise, the seeds 

 of various weeds, and also, involuntarily. Hats, Mice, Cockroaches, and smaller 

 vermin. The origin of tribes may often be inferred from the species of 

 plants and of domesticated and other animals found to have accompanied 

 them.^ 



1 On this general subject consult Wallace's Island Life. 



