Evidently we have no idea at all of what the theory of evolution is about. It takes MILLIONS of years for a change to become obvious. The changes are so tiny that they can't be seen at all for hundreds and thousands of years.
That is because if all the animals start growing halfly may be there will be imbalance in the biological system.
u can see the evolution of humans by an monkey that is because there was once upon a time when it had some disturbance in its growth system  that is the way humans came in to picture.
I think there is no animal who r born half.... but earthworm n other organisms of such kind can live even after they r cut into different halfy.

Hope it helps u!! :) ...