CASES OF ADAPTATION 139 



of animal heat. Another advantage would be, that these 

 muscles being larger proportionally there would be less ex- 

 posure of the internal organs to the extreme cold. 



The result of this interesting experiment is almost conclu- 

 sive as to the reality of natural selection. In this case those 

 which actually survived one of nature's most common tests — 

 exposure to severe storms — and which must be presumed to 

 have been the '' fittest " at that particular time and place, were 

 found to differ in just such characters, and in 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 con- 

 stantly 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 



