In response to a request from George Robert Waterhouse for advice on classification, Darwin replied that it properly "consists in grouping beings according to their actual "relationship", ie their consanguinity, or descent from common stocks". He followed this up with another letter expressing belief that "all the orders, families & genera amongst the Mammals are merely artificial terms highly useful to show the relationship of those members of the series, "which have not become extinct"", before cautiously asking for the letter to be returned. Waterhouse was influenced by Owen and in a paper attacked such heresies, setting his species in the symbolic circles of the Quinarian system, not hereditary trees. Darwin sent a sharp response about these "vicious circles".