In all my years of squirrel-watching, I have never before seen a white squirrel. I spotted this one in South Croydon Recreation Ground; I was on my way to catch a train, so alas I didn't have time to get a better photo. He was chasing a grey around the park when I saw him, apparently fitting in with his social group without a problem. (I'm assuming 'he', as a male squirrel will chase another male in competition or a female for mating, but females are generally pursued rather than pursuer).
Albinism affects mammals, birds, reptiles, amphibians and fish, suggesting it could date back as far as the Silurian, some 450 million years ago. It is the result of a recessive gene, and has a low rate of incidence. Albino animals can be quite healthy, although the disruption of natural camouflage can be a severe disadvantage in the wild. This urban albino squirrel will probably have no particular problems in his life, as predatory birds are rarely a problem for squirrels who live in large cities, and the most common cause of death is road accidents. I hope I will have a chance to see him again next time I'm in London.