74 BIOLOGICAL LECTURES. 



What conclusions can we draw from this pecuhar distribu- 

 tion ? As we have seen, the representatives of each genus on 

 the different islands are very closely related to each other, and 

 certainly have a common origin. The Galapagos Archipelago 

 is comparable to a planetary system.^ The islands form an 

 harmonic group and so do the planets. The planets at a former 

 period were united with each other, and so must have been the 

 islands. Thus only the harmonic distribution of fauna and 

 flora can be explained. It is this consideration which forced 

 me to establish the doctrine of the continental origin of the 

 Galapagos Islands. We cannot explain the harmonic distribu- 

 tion on the theory of oceanic origin of the group, which so far 

 has been adopted. But let us accept this theory for a moment. 

 There must have been a time, shortly after the islands had 

 been lifted out from the ocean, when not a single land-organism 

 existed on them. By and by stragglers from different regions 

 were landed there, and the islands were peopled. But it is 

 impossible that by these accidental colonists the harmony could 

 be produced which we find there. 



Let us now consider the subsidence theory. At a former 

 period these islands were connected with each other, forming a 

 single large island, which itself at a still earlier time was united 

 with the continent, probably with Central America and the 

 West Indies. 2 When this large island was not yet broken up 

 into a series of smaller islands, the number of species must 

 have been very much smaller ; probably there was only one 

 species of Nesomimus, of Certhidia, of Tropidurus, of the 

 Land Tortoise, and so on. Through isolation into single 

 islands the peculiar differentiation of the species began ; an 

 originally single species was differentiated in many different 

 forms ; every, or nearly every, island developed its peculiar 

 races. We still see to-day that islands which are closely to- 



1 " I have said that the Galapagos Archipelago might be called a satellite 

 attached to America, but it should rather be called a group of satellites, physically 

 similar, organically distinct, yet intimately related to each other, and all related on 

 a marked, though much lesser degree, to the great American continent." Darwin : 

 A Naturalist's Voyage, 1845, P- 3^2. 



2 The discovery of land-birds on Cocos Island, which are intermediate between 

 West Indian and Galapagos forms, is very interesting and important. 



