vm 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 

 enumerate briefly the exact causes which must have been at 

 work in bringing about the changes in the rabbits of Porto 

 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. 

 It 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 

 rabbits from which Darwin thinks they were undoubtedly 

 derived. Their numerous enemies would at first capture 

 the larger, more bulky, and slower-moving individuals, then 

 the white or black specimens, who would be more easily 

 seen and pounced upon. This process, continuously acting 

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

 dusky-coloured race. The continuous attack persisting, the 

 size would be again reduced, and the most agile and rapid 

 in movement would alone survive. Thereafter, the nocturnal 

 habit would be acquired by the day-feeders being almost 

 exterminated, and owls would probably alone remain as 



K 



