180 MATTHEW— RECENT DISCOVERIES OF 



nental bridges to account for the \\'est Indian mammals, it is not 

 one but several bridges that they would indicate, each lasting only 

 long enough to permit the passage of one or two of the least migra- 

 tory elements of the continental faunas, and the later ones so 

 arranged as not to disturb the isolation of the types which had 

 already reached the islands. If continental connections be de- 

 manded, the chinchillids call for a Pliocene bridge via the lesser 

 Antilles, not extending west of Porto Rico. The Capromys group 

 calls for a Central American bridge in the late Pliocene or Pleisto- 

 cene, while the ground sloths would demand a Central American 

 bridge in the late Miocene or early Pliocene and subsequent isola- 

 ticui. The giant tortoises similarly would call for connection with 

 the Galapagos islands and isolation since IMiocene or early Pliocene, 

 while the terrapin demands a Pleistocene bridge with Florida im- 

 passable for everything but one terrapin. The two insectivora simi- 

 larly would call for an Oligocene connection with North America 

 and subsequent isolation. Such conclusions seem to me inconsistent 

 and improbable. 



I do not think that one should trust blindly in these indications 

 of former geographic relations from faunal affinities. But I do 

 think that so far as they are of value, they should be fully and fairly 

 presented. It will not do to pick certain points that may fit in with 

 a particular theory, and gloss over or ignore discrepancies ; nor do 

 I see much profit in inventing elaborate hypotheses depending upon 

 a series of unknown factors or unprovable assumptions to account 

 for discrepancies. One may of course say that we do not know 

 what the Tertiary fauna of Central America was like, and that it 

 may have contained just the necessary elements to account for the 

 West Indian fauna through a Pliocene connection. But that merely 

 transfers the difficulties that confront one in attempting to work 

 out any solution that conforms with all the faunal indications. It 

 shifts the problem from the West Indies to Central America. It does 

 not solve it. 



The only explanation that seems to me conformant with all the 

 data, physiographic, geologic and faunal, is that the islands have 

 been populated by colonization through storms and ocean drift with- 

 out land connection with the continents, but aided by extension of 



