350 



C. SKOTTSBERG 



It seems to me that the wide acceptance of land bridges by conchologists is chiefly 

 due to three reasons: (i) our almost complete ignorance of the means of disi)ersal of 

 snails, (2) our lack of knowledge of the speed of speciation in snails, and (3) faulty classi- 

 fication, i)articularly generic classification. A. Gulick has already directed attention to the 

 ])resence of snails on most oceanic islands. 'J'hey were unquestionably carried there by 

 some unknown means of transjiortation. . . . To me it seems incomparably simpler to 

 assume a still unknown method of transportation than a land bridge that is unsupported 

 by any other fact. 



Simple, no doubt, but we cannot get away from the problem by an "ignoramus". 



It is eas\' to under.stand that the presence of fairly large forms of land-shells 

 on distant islands has caused a good deal of trouble. ZIMMERMAN tried to find 

 a wa\' out of the difficulties. 



It has been said that large snails such at the Hawaiian achatinellids and amastrids 

 are particularly unsuited for overseas distribution. However, if we approach the problem 

 differentlv, different conclusions may be reached. If, as I believe, the large Hawaiian 

 snails have evolved from small or minute ancestors, then the argument based upon 

 their large si/e loses its weight. However, if small snails can be distributed overseas, 

 then what is to j)revent eggs or tiny, immature specimens of large species from being 

 similarly transported? (I.e. 61). 



In passing, ZiMMKRMAX quotes H. B. 13.\KKR, who thought that land-shells 

 are carried along by migratory birds. After these speculations it is refreshing to 

 read Hrvax (^o. 9): 



The j)resence of certain kinds of plants and animals found in Hawaii and related 

 to species in the southwest Pacific is hard to explain by any known means of drift, 

 either over the sea or through the air. Land snails constitute one such group. These 

 mollusks, whi( h breathe directly from the air, would drown in water, particularly salt 

 water; yet they must have moisture. They cannot stand long exposure to the sun, but 

 live on the leaves and trunks of forest ])lants or beneath fallen leaves and trash on the 

 ground. How did their ancestors reach Hawaii if they could neither swim nor drift? 



( ^f^poju'iits ai^^aiiist the doi^iua of colonization across tJic oceans. 



Many biogeographcrs have arrived at the conclusion that the natural dispersal 

 agents cannot be made responsible for the distribution of all kinds of biota across 

 very wide expanses of open water. Most of the authors are botanists, which is sur- 

 prising because the chances should be greater for seeds and spores than for eggs 

 or individuals of delicate creatures. To ask for land bridges, or for extension of 

 continental margins later submerged l)ut leaving behind land fragments, is to refuse 

 to accept overseas migration as the only possible means of colonization. Conse- 

 (piently, a number of authors have already been quoted in the chapter on the 

 history of the Pacific basin, (iooD, Ikmsciikk, WTij-f, Cami'BELL, etc., as well 

 as my own contributions to the discussion, the latest in 195 1 [2^8). I shall add 

 here that I never disclaimed every possibility of migration over wide expanses 

 of water; see 226 and 2:12. X'arious writers have compared isolated peaks on 

 continents with oceanic islands and have stated that under present conditions an 

 exchange of biota is improbable. \\\x SlKKMS expressed his opinion when dealing 

 with the Malaysian mountain flora, in very plain terms [2^8)\ too many facts 



