An example and small photograph of isthmus?

Panama. A narrow strip of land linking to larger areas such as South America and the northern part of Central America.

Can´t paste photos because system thinks I am copying the answer from elsewhere.

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.        

answer; biological;