In the realm of mental disease any finger-post which points a way through the entanglements of ætiology is worthy of consideration. Present-day tendency is to magnify external at the expense of internal causes, a cart-before-the-horse policy which must bring confusion. Comparative lunacy endeavours at least to reverse the position and to start straight by tracing the affinity of form which exists between national brain types and their insane corruptions, or by showing variations in the relative incidence of disease forms and co-ordinating these with dominant social or biological factors.