Wallace • The Debt of Science to Darwin 



275 



istic of almost every separate field or 

 bank, or hillside, or wood throughout 

 our land, is the result of a most com- 

 plex and delicate balance of organic 

 forces— the final outcome for the time 

 being of the constant struggle of plants 

 and animals to maintain their exist- 

 ence. 



GEOGRAPHICAL DISTRIBUTION AND 

 DISPERSAL OF ORGANISMS 



Another valuable set of experi- 

 ments and observations are those bear- 

 ing on the geographical distribution of 

 animals and plants— a branch of natu- 

 ral historv which, under the old idea 

 of special creation, had no scientific 

 existence. It is to Darwin that we owe 

 the establishment of the distinction of 

 oceanic from continental islands. By a 

 laborious research into all the accounts 

 of old vovages, he ascertained that none 

 of the islands of the great oceans very 

 remote from land possessed either land 

 mammaha or amphibia when first 

 visited, and on examination it is found 

 that all these islands are either of vol- 

 canic origin or consist of coral reefs, 

 and are therefore presumably of com- 

 paratively 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 orig- 

 inally reached the islands, and the 

 comparative powers different groups 

 of plants and animals possess of tra- 

 versing a wide extent of ocean. 



With this view he made numerous 

 observations and some ingenious ex- 

 periments. He endeavored 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. Seeds 

 might easily be carried 1000 miles, and 

 in very exceptional cases even 3000 

 miles, and still grow. Seeds that have 

 passed through the bodies of birds 

 germinate freely, and thus birds may 

 carry plants from island to island. It 

 was also found that small portions of 

 aquatic plants were often entangled in 

 the feet of birds, and to these as well 

 as to the feet themselves, molluscs or 

 their eggs were found to be attached, 

 furnishing a mode of distribution for 

 such organisms. 



Our space will not permit us to do 

 more than advert to the numerous ex- 

 planations and suggestions with which 

 the Origin of Species abounds, such as, 

 for example, the strange fact of so 

 many of the beetles of Madeira being 

 wingless, while the same species, or 

 their near allies on the continent of 

 Europe, have full power of flight, and 

 that this is not due to any direct action 

 of climate or physical conditions is 

 proved by the equally curious fact that 

 such species of insects as have wings in 

 Madeira, have them rather larger than 

 usual. 



THE DESCENT OF MAN 

 AND LATER WORKS 



We must, however, pass on to the 

 great and important work. The Descent 

 of Man and on Selection in Relation 

 to Sex, which abounds in strange facts 

 and suggestive explanations. None of 

 Danvin's works has excited greater in- 

 terest or more bitter controversy than 

 that on man. 



Observation and experiment were 

 the delight and relaxation of Darwin's 

 life and he now continued to supple- 

 ment those numerous researches on 

 plants we have already referred to. A 

 new edition of an earlier work on the 

 Movements of Climbing Plants ap- 



