In a Chicago Tribune article from 1-5-1913 (I'll link to in comments) a news analyst reports that the "Color Line" in 'cosmopolitan London' was being crossed by African Americans. It was a supposed social problem , this racial tolerance in England, and was defended by what the anonymous reporter called 'the Man To Brother argument'. I have no idea what the writer is referring to.
It's 'men and brother' argument. It's a reference to the famous 18th century anti-slavery image of a kneeling black man in chains, with the slogan 'Am I not a man and a brother?' printed above it. In this context (contrasting the letters of the '"men and brother" type of argument' with the letters that 'regard the matter with seriousness') I'd read it as a way of dismissing the people that disagree with them as a bunch of bleeding-heart do-gooders that care more about 'principles' than The Facts On The Ground (as seen by the author obviously).