Darwin

"Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to the distant futurity"

This is the concept of natural selection and that the fittest will survive at the end and will give their traits and pass them down to the next generation and the next generation.  We see that the fittest will pass their genes because they will be the ones that survive.  The species that survive are the ones that have the genes that are necessary to live and get away from predators or be able to jump, talk, camouflage, any thing necessary to survive.  In this sentence Darwin is saying that there will always be one species that will be superior to another and thus their will always be a species that will defeat another and pass it down to the next generation.  The trait that will be passed down from the strongest link will be the exact same traits, there will be nothing different about it and thus explains why evolution in the beginning took a long time.