The Beauty of Sorrow and The Survival of the Fittest

Both of these items commence with a point and an argument and then illustrate what is meant in terms of narrative. When I hear of the survival of the fittest it makes my hair stand on end. This is because I cannot imagine myself being 'fit'. I fully realize that being fit is a popular ambition nowadays, and it does seem to mean something to those who make various efforts to become fit and if possible to stay fit. I believe it has something to do with exercising. Why will this notion not fit into my head and be content to stay there? When I take a ten mile walk at my present age I don't feel fit. On the contrary, I fancy a rest, a bite to eat and maybe a drink of water. (Beginning of 'The Beauty of Sorrow')