Answer: The phrase that is acting as the subject is "the naturalist".
Explanation: The subject of a sentence is the person or thing that performs an action or expresses a state. It can be a noun, a noun phrase or a pronoun. Moreover, the verb is the part of the sentence that indicates what the subject is doing or feeling. In other words, the verb provides more information about the subject. In the case of the sentence provided, the subject is "the naturalist", which is a noun phrase, since the verb is making reference to an action that "the naturalist" carried out ("realized").
