He had to make her understand that he could not join his lot with her,--chiefly indeed because his heart was elsewhere, a reason on which he could hardly insist because she could allege that she had a prior right to his heart;--but also because her antecedents had been such as to cause all his friends to warn him against such a marriage. 
He had to make her understand that he could not marry her - chiefly indeed because his heart was elsewhere - but also because her history caused all his friends to warn him against such a marriage. 