What is meant by"survival of the fittest" when referring to natural selection

Darwin theorized that animals (such as finches) would mate with the most suitable mate to produced offspring worthy of in-taking the environment it was born unto better than past generations. So, yeah, it's the animal's instincts that lead it to better survival.

Pe = 1/2k(x)^2  and .05n/m = .05x10^-9m = x 

Answermake them get very hot and over time wear them out;