ix THE DEBT OF SCIENCE TO DARWIN 469 



of special creations, had no scientific existence. It is to 

 Darwin that we owe the establishment of the distinction of 

 oceanic from continental islands, while he first showed us the 

 various modes by which the former class of islands have been 

 stocked with life. By a laborious research in all the accounts 

 of old voyages, he ascertained that none of the islands of the 

 great oceans very remote from land possessed either land 

 mammalia or amphibia when first visited ; and on examina- 

 tion it is found that all these islands are either of volcanic 

 origin or consist of coral reefs, and are therefore presumably 

 of comparatively recent independent origin, not portions of 

 submerged continents, as they were formerly supposed to be. 

 Yet these same islands are fairly stocked with plants, insects, 

 land-shells, birds, and often with reptiles, more particularly 

 lizards, usually of peculiar species, and it thus becomes 

 important to ascertain how these organisms originally reached 

 the islands, and the comparative powers different groups of 

 plants and animals possess of traversing a wide extent of ocean. 

 With this view he made numerous observations and some 

 ingenious experiments. He endeavoured to ascertain how 

 long different kinds of seeds will resist the action of salt 

 water without losing their vitality, and the result showed 

 that a large number of seeds will float a month without 

 injury, while some few survived an immersion of one hundred 

 and thirty-seven days. Now, as ocean currents flow on the 

 average thirty-three miles a day, seeds might easily be carried 

 1000 miles, and in very exceptional cases even 3000 miles, 

 and still grow. Again, it is known that drift-timber is often 

 carried enormous distances, and some of the inhabitants of 

 the remote coral-islands of the Pacific obtain wood by this 

 means, as well as stones fastened among the roots. Now, 

 Darwin examined torn -up trees, and found that stones are 

 often inclosed by the roots growing round them so as to leave 

 closed cavities containing earth behind; and from a small 

 portion of earth thus completely inclosed, he raised three 

 dicotyledonous plants. Again, the seeds that have passed 

 through the bodies of birds germinate freely, and thus birds 

 may carry plants from island to island. Earth often adheres 

 to the feet of aquatic and wading birds, and these migrate to 

 enormous distances and visit the remotest islands, and from 



