Although a fish can take in oxygen, they can only take in oxygen from water; as evident by water's molecular confguration (H)
When the fish goes on land, their gills collapse, causing them to suffocate. This is due to the fact that their gills are not suitably made for taking in air. A high surface area is also needed for aquatic animals in order to take in oxygen, and when they go to land, these things all of these things collapse together, blocking their way of taking in oxygen.
Duality of Human nature refers to the dual human nature of human beings. This can be seen in everyday life, for example when we meet people who seem to be kind and friendly, were as in reality they could be manipulative and sly.