Thread: Problem in simple evolution simulator..

Hello,
I am creating a very simple evolution simulator in Processing.
It is a simple game where you must move the player around some terrain (circles that I call rocks).
There are 50 randomly generated creatures (brains) that each try to play this game.
At the end of a generation (50 creatures), half of the creatures (the worst performing ones) are killed off and the other half each get one slightly randomized offspring.
Here is the problem :
Sometimes, the best creature gets worse from one generation to the other. I don't know if it's getting randomized or getting killed, but I do know it shouldn't happen.
I've been trying to fix this, for days and I haven't found any solution!

The "creatures" try to go to the top right of the screen.
They are sorted based on the distance from the top right.

Every step of the sort, I output in the console each creature's distance. So it would look like this, but with more numbers.
980, 560, 320, 1080
560, 320, 980, 1080
320, 560, 980, 1080

How ever, in the next generation, for some reason there is a chance the 320 would be gone and the 560 takes it's place..

That's what I'm trying to find out!
What is making the best creature worst/die?