Nature tends to weed out retardation amongst animals, but it does occur. Animals with any deficiency, in the wild, are shunned. If they can't defend or feed themselves, they die. It's natures way of a particular species remaining strong, thus increasing that species chance of survival.
In caveman days, the weaker, or deformed (be it physically or mentally) were left behind so that they didn't dilute the gene pool. If I had a retarded or handicapped child, I would certainly love it and care for him or her, but if you look at it logically, doing so goes against nature.