...is the narrator's (or JA's) conclusion that "elasticity of mind" is "from nature alone". If this were one of the usual moralizing 19th-century tracts, the conclusion would be that this quality, like all other character traits, was a matter of will-power, and that the sufferer could develop it if only s/he tried hard enough. Instead, JA evidently concluded that admirable as this virtue was, you either had it or you didn't, and presumably you shouldn't beat yourself up if you didn't.