This system of locking cannot function in a crop pathosystem in which the host population has genetic uniformity. A crop pathosystem is usually the equivalent of every door in the town having the same lock, and every householder having the same key which fits every lock. A system of locking is ruined by uniformity, and this is exactly what we have achieved when protecting our genetically uniform crops with vertical resistance. It also explains why vertical resistance is temporary resistance in agriculture. This type of error is called sub-optimization and it results from working at too low a systems level. The system of locking is an emergent property that is observable only at the systems level of the pathosystem. Comparable biological emergents are the schooling of fish, and the flocking of birds, which cannot be observed at any systems level below that of the population. The n/2 model is also the most important hypothesis to emanate from the concept of the pathosystem. It can also be argued that the gene-for-gene relationship must function on a basis of heterogeneity in the wild pathosystem because the gross instability of the 'boom and bust' of modern plant breeding would have no evolutionary survival value.