DISTRIBUTION OF SPECIES 



than for others, but at any particular time, the probabiHty of a successful 

 crossing by any organism is small. But, chance is the major factor in de- 

 termining that a crossing is made, and so less probable organisms may 

 succeed while more probable ones fail. This is comparable to a lottery 

 (whence the term sweepstakes) in which a person who holds only one 

 ticket may win, while a person who holds many tickets may lose. Finally, 

 a sweepstakes is likely to be a one-way route, in contrast to corridors and 

 filter bridges. Island life is commonly established via the sweepstakes 

 method, and because of this it is likely to be rather unbalanced. 



Simpson has summarized the results of the sweepstakes colonization of 

 Madagascar in Figure 112. Lions, elephants, apes, antelopes, and zebras 

 are selected to typify animals which cannot cross the Mozambique Chan- 

 nel to Madagascar either because they are too large for the natural rafts, 

 or because they do not approach the seashore for ecological reasons. 

 They do not hold sweepstakes tickets. Many animals, however, hold 

 tickets as there is no apparent reason why they could not cross the chan- 

 nel as easily as those that have done so, yet these "disappointed ticket- 

 holders" have not crossed. Out of the multitudes of ticket-holders in the 

 African fauna, only a few have "won," and these have been determined 

 by chance, not by the characteristics of the animals. These are typified 

 by some mice; by certain cat-like carnivores of the family Viverridae, 

 including the fossa which is illustrated; by lemurs and some other primi- 

 tive Primates; by the tenrec, a peculiar insectivore; and by a pigmy hip- 

 popotamus which may have crossed the channel by swimming. These are 

 all types which are represented in Africa by widely diversified groups. 



AFR.I C A -- 



NOT HOLDERS 



OF 



TICKETS 



JSt 



EICPHANT$ 



DISAPPOINTED, 

 TICKET 

 HOLDEI^S 



THE 

 WINNING TICKETS 



PALEOCENE-TENRECS 



EOCENE LEMURS 



OLIGOCENE-FOSSAS 

 MIOCENE ••••MICE 

 PLIOCENE- NO DRAWING 

 PIEISTOCENE HIPPOPOTAMUS 



I 



sweepstak^es 



FiGxmE 112. The African-Malagasy Sweepstakes. (From Simpson, /. Washington 

 Acad. Sci., V. 30, 1940. ) 



319 



