DISTRIBUTION OF SPECIES 



population, which can be regarded as an endemic, remains. With the ex- 

 tinction of this remnant, the history of the group comes to an end. 



DISCONTINUOUS DISTRIBUTION AND BRIDGES 



It is in connection with this type of history that the problem of widely 

 discontinuous distributions must be understood. In any particular case, 

 discontinuity may be brought about by extinction in the intermediate 

 parts of a wide distribution, or by the bridging of a barrier between two 

 distinct areas, or by a combination of the two. This brings us to the prob- 

 lem of how barriers may be bridged. Simpson has analyzed this problem 

 and has classified bridges into three types: corridors, filter bridges, and 

 sweepstakes bridges. A corridor is a broadly continuous connection, exist- 

 ing over a long period of time, so that it permits an extensive interchange 

 of the floras and faunas of the connected regions. Such a connection now 

 exists between Europe and Asia, and these two accordingly comprise a 

 single biogeographic region. A filter bridge is more temporary in duration, 

 and more restricted in extent. Conditions are more uniform upon it, with 

 the result that it "filters" the flora and fauna which might use it; only 

 those with appropriate characteristics can pass. The Bering Strait acted 

 as a filter bridge for mammals during the Pliocene period. Only those 

 mammals could cross which were capable of making a rapid crossing and 

 of withstanding cold weather. A sweepstakes bridge, on the other hand, 

 does not involve migration across a land connection, but rather it depends 

 upon accidental transportation in the absence of any real connection. Cor- 

 ridors and filter bridges should operate equally well in either direction, 

 and should cause the exchange of numerous forms; but a sweepstakes 

 bridge should operate only in one direction, and only a few forms should 

 succeed in crossing it. It is a one chance-in-a-million phenomenon. But 

 this may allow many chances indeed if time be available on a geological 

 scale. 



Corridors. Corridors are most striking after they no longer exist, that 

 is, after geological events have separated land masses which once were 

 continuous. Simpson has pointed out that New Mexico and Florida can 

 be regarded as being connected by a corridor at present. As expected, 

 they share most of their major groups and a large number of genera, but 

 species are likely to be different because the climatic conditions of the 

 two states require rather different adaptations. One gives little thought 

 to such a situation as a "bridge," simply because the question never arises 

 as to why the areas concerned should not have a substantially similar 

 flora and fauna. But it is perfectly possible that they might be separated 

 in future ages by, for example, an inland sea occupying the present Missis- 

 sippi drainage basin. And then paleontologists might use the fossils of 

 our time, or common genera of their own time, to prove that once a cor- 

 ridor existed between New Mexico and Florida. 



Such is the situation with respect to eastern Asia and eastern North 

 America. During much of the earth's history, Asia and North America 

 were broadly connected across the north Pacific. Early in the Tertiary, 



S15 



