CASES OF ADAPTATION 129 



such moderate proportions as have been found to occur 



: constantly in all the commoner species of birds, as well as of 

 all other animals. It proves also that such small variations 

 are, as Professor Lloyd Morgan terms it, of " survival value," 

 a fact which is constantly denied on purely theoretical 



| grounds. 



It will perhaps make the subject a little clearer if I here 



I enumerate briefly the exact causes which must have been at 

 work in bringing about the changes in the rabbits of Porto 



I Santo during the four and a half centuries that had elapsed 

 from the time they were turned loose upon the island to the 



' period when Darwin obtained his specimens. The island 

 has an area of about 20 square miles; it is very hilly, of 



'volcanic origin, with a dry climate and scanty vegetation. 



jit is about 26 miles from Madeira, 400 from Africa, and 

 250 from the Canary Islands. The powers of increase of 

 rabbits being so great, and the island being at that time 

 uninhabited, they would certainly in a very few years have 

 increased to so great a multitude as to consume all the 

 available vegetation. As they approached to these numbers, 

 and were obliged to expose themselves in the daily search 

 for food, many birds of prey from the larger island, and 

 probably others from the Canaries and from Africa — hawks, 

 buzzards, falcons, and owls — would flock to this hitherto 

 desert island to feed upon them, and would rapidly reduce 

 their numbers. 



Up to this time, perhaps not more than a dozen or 

 twenty years from their first introduction, they would have 

 varied in size and colour as do the common domesticated 

 'abbits from which Darwin thinks they were undoubtedly 

 (derived. Their numerous enemies would at first capture 

 :he larger, more bulky, and slower-moving individuals, then 

 :he white or black specimens, who would be more easily 

 ;een and pounced upon. This process, continuously acting 

 or a few generations, would result in a smaller and more 

 lusky-coloured race. The continuous attack persisting, the 

 ;ize would be again reduced, and the most agile and rapid 

 n movement would alone survive. Thereafter, the nocturnal 

 labit would be acquired by the day-feeders being almost 

 exterminated, and owls would probably alone remain as 



K 



