Throughout african history, the search for fertile soil has led to many migrations. what was a result of these migrations?

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

when your a dog and you need a bone.   go and eat a rabbit