Sentence:  Mark hardly knew whether to believe this or not. He already began to suspect that Roswell was something of a humbug, and though it was not in his <WSD>nature</WSD> to form a causeless dislike, he certainly did not feel disposed to like Roswell. 
Meaning of the word marked with <WSD> tags:  The innate characteristics of a thing. What something will tend by its own constitution, to be or do. Distinct from what might be expected or intended.