290 INTRODUCTION TO EVOLUTION 



rice rats. As noted before, the latter rodents were evidently the only terres- 

 trial mammals to reach the archipelago prior to the coming of man. Inten- 

 tionally and unintentionally man has introduced cattle, horses, donkeys, 

 pigs, dogs, goats, and black rats. These have run wild over the islands. 

 That they have found the new home congenial is affirmed by the statement 

 of Lack (1947): "The characteristic music of the Galapagos forest is not 

 the song of birds but the braying of donkeys." 



It is particularly significant that before these introduced species arrived 

 there were no large, herbivorous mammals on the islands. The environ- 

 mental niches usually filled by grazing and browsing mammals (e.g., deer, 

 moose, antelope, and so on) were not filled by mammals. Probably it was 

 owing to this fact that the reptiles were able to undergo the remarkable 

 developments we have just noted. Darwin was impressed with this thought: 

 "When we remember the well-beaten paths made by the thousands of huge 

 tortoises — the many turtles — the great warrens of the terrestrial [iguana] — 

 and the groups of the marine species basking on the coast-rocks of every 

 island — we must admit that there is no other quarter of the world where 

 this Order replaces the herbivorous mammalia in so extraordinary a man- 

 ner." It was as though this isolated region, being free of mammalian com- 

 petitors, afforded the reptiles one last chance for an adaptive radiation 

 recalling, though dimly, the Mesozoic "golden age" of reptiles. 



Other Animals and Plants 



The small number of land insects and of land molluscs, as well as the 

 great gaps in the expected types of plants, also bear witness to the dis- 

 harmonic nature of the fauna and flora. Many widespread plant groups are 

 notably absent, among them conifers and palms, as well as several impor- 

 tant families characteristic of tropical America. The islands are old enough 

 so that the environmental niches left vacant have been filled, at least par- 

 tially, by plants which did succeed in reaching the islands. Thus, the typi- 

 cally low-growing prickly pear cactus has, on Galapagos, become a tree. 

 We have noted previously (p. 283) that the filling of environmental niches 

 by forms other than those normally filling them on continents is charac- 

 teristic of floras and faunas of oceanic islands. 



How Were the Islands Populated? 



How did plants and animals reach the Galapagos Islands? Were the 

 latter ever connected to America? At the present time no answer can be 



