286 INTRODUCTION TO EVOLUTION 



day. Since a sail was used the steady trade winds augmented motive force 

 provided by ocean currents. 



Probably reptiles are more likely to withstand the vicissitudes of travel 

 by natural rafts than are mammals, though the latter have occasionally 

 been seen on rafts of this type. In this connection we may well recall that 

 man has, from prehistoric times, traveled from island to island and from 

 continent to island by boats and ships. He has intentionally transported 

 \j some land animals in his boats, but in addition to these he has undoubtedly 

 transported unintentionally an indeterminable number as stowaways. This 

 fact adds difficulty to the problem of determining which mammals, for 

 example, inhabited a given island before the coming of man. In the case of 

 the Galapagos Islands there seems no doubt that rice rats, small rodents of 

 South American relationship, were present before the advent of human 

 visitors; but Gulick (1932) has concluded that "this may count as the only 

 unequivocal instance of a mammal that has preceded man across an appre- 

 ciable stretch of ocean." If such is the case natural rafts need have ac- 

 counted for but Httle mammalian distribution, even that of small mammals. 

 We noted earlier that large land mammals, for which natural rafts would 

 afford inadequate means of transportation, are conspicuously absent from 

 oceanic islands while conspicuously present on continental islands. 



We note that all the means of dispersal just discussed are accidental, in- 

 volving a large element of chance. It is significant that chance is just the 

 factor which seems to have been operative in the production of the dis- 

 harmonic floras and faunas characteristic of oceanic islands. The objection 

 may be raised that the means postulated are too meager to have accounted 

 for observed populations on oceanic islands. Yet the original number of 

 immigrants may have been small, and their arrivals widely spaced in time. 

 In the long stretches of geologic time even very improbable events, if they 

 are not impossible, may occur. Thus Zimmerman (1948) concluded with 

 regard to the rich and varied insect population of Hawaii "that over a 

 period of several millions of years, only about 250 overseas stragglers suc- 

 ceeded in becoming established in the several thousand square miles of the 

 Hawaiian Islands — perhaps only one successful colonization per 20,000 

 years!" 



Some of the smaller oceanic islands bear evidence of relatively recent 

 formation. This fact precludes the possibility that they were ever connected 

 to continents or other islands by land bridges. Thus they may be considered 

 to provide test cases of the efficacy of the accidental means of dispersal de- 

 scribed above. One such island in the Pacific is Henderson, located some 



