208 -LV.V.l/.N -V/.'U VOh'h ACADEMY OF SCIENCEfi 



more or less ii(|uatic Imliits and witliin certain limits of distance, it is an 

 apparent ])h3'sical impossibility. The third may be either intentional or 

 accidental and should be considered in connection with the kno\vn custom 

 among Malays and other races, of taming various captured animals and 

 taking them along on sea-voyages. Its application is, of course, limited 

 to distributional anomalies of late Pleistoqpne or modern origin. The 

 last hypothesis, where it traverses the doctrine of the permanence of ocean 

 basins, appears to me unnecessary, as I have failed to (iiid a single in- 

 stance of distribution wliieh cannot reasonably be otherwise explained. 



COXSIDERATIONS AFFECTING PROBABILITIES OF OVER-SEA MIGRATION IN 



SPECIAL CASES 



The prc)l)abilities of over-sea transportation to an oceanic island will 

 o})viously l)e much greater if the island is large, and correspondingly re- 

 duced if it be of small size. The distance from the mainland will greatly 

 reduce the chances of such rafts making a landing, for two reasons : first, 

 the chances of survival of the animals are reduced proportionately to the 

 length of their journey (or rather, in a varying relation, which for con- 

 venience we may consider as a direct proportion) ; second, most rafts will 

 be carried out from one or more points along the coast, but not from all 

 points equally (that is to say, from the mouths of one or more great 

 rivers, where the conditions are favorable, seldom from any of the small 

 rivers). If Me disregard prevalent winds and currents and consider the 

 rafts as drifting out in all directions the probability of their landing on a 

 given island will bo dii-ectly ])roportioned to its length opposite the main- 

 land, inversely to the distance. The probabilities of survival of animals, 

 so far as it depends on the raft holding together, will also be inversely as 

 the 71 umber of days exposure to the sea, hence as the distance. Compar- 

 ing Saint Helena, HOO miles from Africa and 10 miles diameter, with 

 Madagascar, 200 miles from Africa and 1000 miles in length, we see that 

 the probabilities of effecting a colonization would be 100 X 31/2 X 5^/2? 

 or 3025 times greatiT in the case of Madagascar. New Zealand, 800 miles 

 long and 1200 miles from the Australian coast, will receive 8/10 X 1/6 

 X 1/6, or 1/45 as many colonizations as Madagascar, but 80 X 11/12 

 X 11/12 or 67 times as many as Saint Helena. 



I believe that it is to their small size rather than to unfavorable con- 

 ditions for survival that the poverty of fauna, especially of higher verte- 

 ])rates, in the smaller oceanic islands is due. 



The oceanic currents and prevalent winds do. of course, profoundly 

 modify the above generalities in each individual instance. They have 



