So much for the defended areas. With regard to the great routes that connected them, it has been said they were left undefended. By this is meant that the security of ships passing along them was provided for, not by patrols but by escort. The convoy system was adopted, and the theory of that system is that while vessels are on the great routes they are normally liable only to sporadic attack, and they are consequently collected into fleets and furnished with an escort sufficient to repel sporadic attack. In theory, cruiser escort is sufficient, but in practice it was found convenient and economical to assign the duty in part to ships-of-the-line which were going out to join the distant terminal squadron or returning from it for a refit or some other reason; in other words, the system of foreign reliefs was made to work in with the supplementary escort system. Where no such ships were available and the convoys were of great value, or enemy's ships-of-the-line were known to be out, similar units were specially detailed for convoy duty to go and return, but this use of battle units was exceptional. Such a method of dealing with the great routes is the corollary of the idea of defended areas. As those areas were fertile and likely to attract raiding squadrons, so the great routes were infertile, and no enemy could afford to spend squadrons upon them. It is obvious, however, that the system had its weak side, for the mere fact that a convoy was upon a great route tended to attract a squadron, and the comparative immunity of those routes was lost. The danger was provided for to a great extent by the fact that the enemy's ports from which a squadron could issue were all within defended areas and watched by our own squadrons. Still, the guard could not be made impenetrable. There was always the chance of a squadron escaping, and if it escaped towards a critical trade-route, it must be followed. Hence there were times when the convoy system seriously disturbed our dispositions, as, for instance, in the crisis of the Trafalgar campaign, when for a short time our chain of defended areas was broken down by the escape of the Toulon squadron. That escape eventually forced a close concentration on the Western Squadron, but all other considerations apart, it was felt to be impossible to retain the mass for more than two days owing to the fact that the great East and West Indies convoys were approaching, and Villeneuve's return to Ferrol from Martinique exposed them to squadronal attack. It was, in fact, impossible to tell whether the mass had not been forced upon us with this special end in view.