Take the simple example of a deer and a tiger

A typical night in the forest. When carnivores hunt for food and their prey’s run for life.
A deer has to run in order to save its life when predators like the tiger come in search for food where as the tiger also needs to run in order to get food to survive.

Its hard work (in this case speed and effort), the one who runs faster is the survivor.
The faster the deer runs - the luckier it gets and by the same logic - the faster the tiger runs - the luckier it gets.
And also speed is dependent on fitness and knowledge of the terrain both of which require extreme hard work again in the form of regular exercises and learning.

B- This case is an isolated one, where would you find only 1 deer and 1 tiger. The fact is that there are many tigers and many deer’s. And the deer need not be the fastest in order to save itself; it just needs to be relatively faster than the slowest deer. On the other hand the tiger also does not need to be the fastest; again it just needs to be relatively faster than the slowest deer.

Now if we add all carnivores and all their potential food to the list it becomes even easier for a smart deer or a smart tiger to save itself.

Just be faster than the slowest animal and repeat the exercise

Note : from my own blog "http://matmaniac.blogspot.in/2017/11/conversation.html"