[Such] irregularities … do not impair the great theoretical significance of the periodic classification, although for practical purposes the elements have often been subject to a different arrangement." (op. cit. p. 236) What Dobbin and Walker said about different arrangements of the elements is pertinent: there is nothing in this paper ostensibly claiming the inherent superiority of its four-fold classification. Approaches to classification can vary depending on the perspective of relevance at the time.