Pacifier

that is not correct.
of only the fittest species whold have survived there would be only one species left.
"Survival of the fittest" is about all individuals within one species. it is about "niches" to live in. Birds do not compete against cats (according to your theory birds must have been long extinct since cats are "fitter" compared to a bird especially a young one). Birds compete against other birds within their species. If you are fitter then your neighbr bird you will more likely breed and spread your genetic code. Bird and Cats dont compete, they are not even playing the same game.

BTW: the term "survival of the fittest" was not used by darvin, it was used by the journalist Herbert Spencer whos job it was to interpret and explain darwins theories to the masses.
And, as someone said,
"The phrase has the advantage of convincing everybody that they understood what darwin was saying, and it hat the disadvantage of convincing everybody that they understood what darwin was saying "

__________________
"It seems to me that the idea of a personal God is an anthropological concept which I cannot take seriously. I also cannot imagine some will or goal outside the human sphere. Science has been charged with undermining morality, but the charge is unjust. A man's ethical behavior should be based effectually on sympathy, education, and social ties and needs; no religious basis is necessary. Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death
— Albert Einstein