It will be necessary for us, therefore, to determine whether these swords, which have penetrated nearly the whole of Europe except the Iberian peninsula, were carried by trade, by some other form of peaceful penetration, or by conquest. The great suddenness with which some of the types spread, apparently within the space of a few years, for there is little if any modification of form, from the central region to places many hundreds of miles distant, precludes the second of these alternatives, for peaceful penetration by land is a slow process, and we should expect progressive variation of type the farther we pass from the centre. It seems, on the face of it, unlikely that a people, especially a sporting and warlike people like our steppe-folk, would engage in a trade which would provide their neighbours with a weapon, superior to all others available, which they had produced for themselves after generations of experiment; nor is it likely that they would permit their Alpine subjects to do so, even if the fear of the unknown and the dislike of adventure had not been sufficient to prevent these home-loving people from setting out on so adventurous a task, involving, as it sometimes did, the passage across northern seas. Such a practice, then, seems at first sight unlikely, but if the other alternative, the hostile invasion, were true, we should expect to find evidence of the arrival of fresh people in the presence of new types of pottery and fresh burial customs. If we examine the British evidence, we shall find reason to believe that the leaf-shaped swords arrived with a new culture and a fresh element in the population. In a recent paper Mr. O. G. S. Crawford has dealt with this subject, and pointed out that the leaf-shaped bronze swords of the Hallstatt period, our Type G, arrived with an invasion of people who came from Central Europe.[348] Crawford seems to include in this movement all bronze leaf-shaped swords of whatever type, but the evidence on which he depends is only applicable to Type G. It will be well, therefore, for the moment to postpone consideration of the arrival of the Type E swords.