14 INTRODUCTORY. [CHAP. 



to swim straight out to sea in an unknown direction. Moreover, 

 supposing a mammal new to a particular island to have arrived 

 there by swimming, unless it happen to be a pregnant female, or 

 unless another individual of the same species but of the opposite 

 sex should arrive soon after (a most unlikely event), it would in 

 due course die without being able to propagate its kind. And 

 even if it should happen to be a pregnant female, there would be 

 no certainty that its offspring, if but one in number, should be of 

 the opposite sex to its parent. Accordingly, it would seem that 

 the population of islands by mammals that have arrived by swim- 

 ming must be a very rare event indeed. Rafts may be of more 

 importance. Mr Aplin, in the Proc. Zool. Soc. for 1894, mentions 

 that jaguars and pumas are frequently transported by them from 

 one side of the Rio de la Plata to the other; and in the rainy 

 season many are to be seen off the northern coast of Borneo, 

 some of which may be as much as thirty yards in length. Still 

 such rafts are not likely to cross straits, except when there is a 

 current setting from one bank to the other. 



Before leaving this part of the subject, it must be mentioned 

 that the degree of difference between the fauna of an island and 

 that of the adjacent continent, or between the faunas of two 

 islands, affords a most important clue as to the relative date of the 

 land-connection between them. Madagascar, for instance, has a 

 mammalian fauna which although clearly derived from Africa, is 

 yet so different from that now inhabiting the mainland, that the 

 land-connection between the two must have been broken up at an 

 epoch comparatively remote. Ceylon and India present a condition 

 in respect of their faunas intermediate between the last and that of 

 Great Britain as compared to the Continent. In the latter instance, 

 with the exception of a single Irish weasel, all the mammals are 

 identical with species now inhabiting the Continent, thus proving 

 that the connection has been a comparatively recent one ; although 

 the peculiar weasel indicates that the separation between Ireland 

 and Britain has been of sufficient duration to admit of the develop- 

 ment of a distinct specific type in the former country. 



Although large rivers like the Amazon and La Plata un- 

 doubtedly form serious barriers to the migration of mammals, yet 

 these are not so insuperable as might at first sight be supposed, 



