Final answer:
The presence of the porpoise in 'The Sea Devil' is believable within the naturalistic setting of the story, where human control is limited and the action occurs as a result of unpredictable natural forces.
Step-by-step explanation:
In The Sea Devil, the action of the porpoise that influences the struggle in the story can be seen as believable within the context of Naturalism, a literary movement where nature is often depicted as indifferent to human struggles. The author has prepared us for the appearance of the porpoise by setting up a world where human characters are at the mercy of larger natural forces, suggesting that the porpoise's action was not simply a matter of chance but a plausible occurrence within the story's naturalist framework.
By portraying nature as vast, indifferent, and sometimes hostile, in alignment with the Naturalist writers like Crane and London, the author has created an environment where not only is human control limited but events occur due to the interaction with the natural world.
The presence of the porpoise, while unexpected, fits into the larger narrative that emphasizes the unpredictability of nature and its creatures. Thus, when Naturalism's focus on the deterministic qualities of environment and heredity are considered, the appearance of the porpoise can be understood as a probable, if not necessary, part of the story's plot.