A DEFINITION OF EVOLUTION 



include a high proportion of endemics, no doubt because of the rarity of 

 a successful crossing of the water barrier. 



The only understandable basis for these facts is Darwin's hypothesis, 

 that the islands were colonized from the mainland, with the colonists be- 

 coming modified subsequently. As they spread to the various islands of 

 the archipelago, each isolated population was modified independently, 

 with the result that groups of closely related, endemic species were 

 formed. The connection, then, between the various similar species of an 

 archipelago and of the nearest continent is simply heredity. 



Very tersely then, the actual geographical distribution of organisms is 

 readily understandable on the assumption that each group has originated 

 in one of the major regions of the world, then spread to occupy as much 

 space as it could in the face of physical and climatic barriers, and of com- 

 petition from other organisms. The result has been continuous pressure 

 of natural selection, leading to adaptation of the group to a wide variety 

 of circumstances, that is, to evolution. Much of the data of distribution 

 would be anomalous on any other basis. It is little wonder that first-hand 

 experience with so impressive and persuasive a series of facts should have 

 suggested to Darwin the possibility that species are mutable. 



REFERENCES 



Barlow, Lady Nora (Ed.), 1946. "Charles Darwin and the Voyage of die Beagle," 

 Philosophical Library, New York, N.Y. Darwin's granddaughter here presents selec- 

 tions from his letters to his family and from his notebooks on the voyage. 



Darlington, P. J., 1957. "Zoogeography: the Geographical Distribution of Animals," 

 John Wiley & Sons, Inc., New York, N.Y. A readable, thoughtful, and thought- 

 provoking rethinking of the entire field— the first such since Wallace's. 



Darwin, Charles, 1845. "Journal of Researches," 2nd Ed., Appleton & Co., New York, 

 N.Y., and London. The original report of the voyage of the Beagle, and Darwin's 

 "favorite literary child." 



DE Beaufort, L. F., 1951. "Zoogeography of the Land and Inland Waters," Sidgwick 

 & Jackson, London. A useful summary. 



Ekmax, S. p., 1953. "Zoogeography of the Sea," Sidgwick & Jackson, London. Brief 

 but excellent. 



Wallace, Alfred Russel, 1876. "The Geographical Distribution of Animals," Tlic 

 Macnn'llan Co., New York, N.Y. After more than SO vcars, this is still the most fim- 

 damental work in its field. 



26 



