Law of marine deviation being an organic part of English law of contract was equally affected by developments of the doctrine of dependency and independency of promises. The general rule established by the sixteenth century was that mutual covenants were independent unless express words of condition made one covenanted performance dependent upon the other.
Leduc & Co v Ward (1888) 20 QBD 475 per Lord Esher M.R. at p.482:
It was argued that that clause gives liberty to call at any port in the world. Here, again, it is a question of the construction of a mercantile expression used in a mercantile document, and I think that as such the term can have but one meaning, namely, that the ports, liberty to call at which is intended to be given, must be ports which a substantially ports which will be passed on the named voyage.
Margetson v Glynn  1 QB 337 per Lord Esher M.R.:
What is that liberty? It is a liberty with regard to that voyage. It is not liberty to go another voyage, but liberty to do something with regard to that voyage which the ship could not do if the bill of lading had stopped at the words 'from Malaga to Liverpool…
The ship went straight away from the direct course to Liverpool; she went in the opposite direction to Burriana, and [council for the shipowners] was obliged to admit that his argument, if well-founded, would equally allow the ship to go to Constantinople, then into the Black Sea to Sebastopol, and thence round the coast of Africa to Liverpool.