140 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Seh. 



those who beheve that these islands all formerly were con- 

 nected, and formed part of a single large island which, some- 

 time, must have been connected with continental America. 



We can see but little to commend the former view, for the 

 means of dispersal from continent to island or from island to 

 island over the intervening water must, in the nature of things, 

 be but accidental or occasional, and must seem quite inade- 

 quate to account for the wide distribution of these lizards in 

 the archipelago. Again, Wenman and Culpepper islands lie 

 directly in the path of the currents from the other islands, yet 

 both are without Tropiduri. Furthermore, were such means 

 of dispersal sufficient to bring about the wide distribution of 

 these reptiles, we must believe that the interchange of lizards 

 between the islands would result either in preventing differ- 

 entiation on the various islands, or in the transportation of 

 differentiated races from island to island. Thus we should 

 expect to find either one kind of lizard on all the islands, or a 

 tendency toward the distribution of all kinds of lizards to each 

 island. But many of the islands have each its peculiar kind 

 of Tropidurus, and no island has more than one kind. Even 

 in the case of Duncan Island, almost surrounded as it is by 

 other close-lying islands, the evidence all points to complete 

 isolation during a long period of time. 



Origin and History of the Galapagos Islands 



We, therefore, adopt the other theory: that there formerly 

 was a single large island inhabited by one species of Tropi- 

 durus; that through partial and gradual submersion this island 

 became divided into the many islands of the present archi- 

 pelago; that each island after its separation was occupied by 

 those animals which inhabited it before; and that the present 

 fauna of each island is directly descended from its original 

 inhabitants. 



It is probable that the separation of the various islands oc- 

 curred at different times rather than simultaneously. 



If it be admitted that the degree of differentiation in a single 

 group, under conditions such as obtain in these islands, may 

 be regarded as an index to the period of isolation, we may pro- 

 ceed to sketch the history of the archipelago as indicated by 

 the lizards of this genus. 



