BARBOUR, JIATTHEW'S ''CLIMATE AXD EVOLUTION" 7 



capable of withstanding transport by rafts conspicuously absent in the 

 West Indies ; for example, Basiliscus, almost semiaquatic, is absent in the 

 Antilles, as Yaranus is absent upon Madagascar : they were possibly de- 

 rived in common with many other of the islands' original continental 

 inhabitants and have failed to survive. We know that some edentates 

 and a few rodents have become extinct in the West Indies, some within 

 the last half century; why, in some cases, we cannot guess; probably more 

 have gone the same way, of which we have unfortunately no fossil re- 

 mains. So also the boa-constrictor, which has been observed to be carried 

 once to St. Vincent from the Orinoco and which has probably come com- 

 paratively often, has never succeeded in establishing itself. Why some 

 types fail to survive upon islands while others of apparently similar habit 

 flourish is one of the enigmas for which no mite of answer has appeared. 

 Matthew's arguments, which he numbers 4 and 5, may be considered to- 

 gether: "4) Three hundred miles drift would readily reach any of the 

 larger oceanic islands except New Zealand. Assume as one in ten the 

 probability that the raft drifted in such a direction as to reach dry land 

 within three hundred miles. 5) In case such animals reached the island 

 shores and the environment afforded them a favorable opening, the propa- 

 gation of the race would require either two individuals of different sexes 

 or a gravid female. Assume the probability of any of the passengers sur- 

 viving the dangers of landing as one in three (by being drawn in at the 

 mouth of some tidal river or protected inlet), of landing at a point where 

 the environment was sufficiently favorable as one in ten, the chances of 

 two individuals of different sexes being together might be assumed to be 

 one in ten, the alternate of a gravid female as one in five. The chance of 

 the two happening would be 1/10 + 1/5 = 3/10. The chance of the spe- 

 cies obtaining a foothold would then be 3/10 + 1/3 + 1/10 = 1/100.'" 

 He then continues, "If then we allow that ten such cases of natural rafts 

 far out at sea have been reported, we may concede that 1000 have prob- 

 ably occurred in three centuries and 30,000,000 during the Cenozoic. Of 

 these rafts, only 3,000,000 will have had living mammals upon them ; of 

 these only 30,000 will have reached land, and in only 300 of these cases 

 will the species have established a foothold. This is quite sufficient to 

 cover the dozen or two cases of Mammalia on the larger oceanic islands, 

 "I have considered the case only in relation to small mammals. With 

 reptiles and invertebrates, the probabilities in the case vary widely in 

 different groups, but in almost every instance they would be considerably 

 greater than with mammals. The chance for transportation and survival 

 would be larger and the geologic time limit in many instances much 

 longer. Wind, birds, small floating drift and other methods of accidental 



