Hello there, today I'm bringing you a small cartoon by a very smart guy. This cartoon belongs to a series called "The Far Side" and was drawn by Gary Larson. 
As you should know, darwinian natural selection deffends the survival of the fittest, being overcentered in the inter-species competition, but, nowadays, neo-darwinian natural selection is all about the best traits (the best alleles in each gene) and the way they are passed down to the descendants, centered in the intra-species competition.
In this case, darwinians would deffend that the carnivores were stronger, and that led to the end of the unicorns, but, in other and, neo-darwinians would say that the unicorns did not posses the traits to survive the stress caused by the carnivores. 
We can also look at this cartoon and notice a problem that has been present in all the planet for some centuries. The problem is the way species interact with each other and, what could happen if we change some individuals from habitat. But I'll leave that for another day.