This might have been the cause of not being exposed to this specific pollen during his years in Chicago. Allergic reactions can develop when the stimuli presented even when its not contagious causes the immune system to trigger and act in a defensive stance. This delayed effect may have been the cause of the sudden expression of the immune system.
It supports it by saying that there was once only one mammal but then it reproduced and since different places have different climates they adapted or evolved to the climate changing their form and shape.
