It's bad writing.
What Cooper is trying to say is that
The deportment (behavior) of the 'highly bred' (meaning people who are raised in luxury and learn elegant behavior from their youth) is marked by 'a refined simplicity'.
Those born to a less elevated station should aim to achieve'a considerate humanity' (meaning 'consideration for others as human beings') in their deportment.
That is, ordinary people should not mimic the behavior of the rich, which can only be carried off by people 'bred' to it, but should content themselves with being decent human beings.
By way of analogy, replace 'deportment' with 'language': "Only those who have read and spoken the Standard Language from birth can master all the subtleties of a language; speakers of non-standard dialects and other languages should aim at clarity and precision." There's some truth in it, but it's a very condescending and ethnocentric attitude.