This necessary specialisation and detail training has large consequences for the growth of technology as well as for its custody and transmission. It follows that a large and widely diversified industrial scheme is impossible except in a community of some size,--large enough to support a number and variety of special occupations. In effect, substantial gains in industrial insight and proficiency can apparently be worked out only through such close and sustained attention to a given line of work as can be given only within the lines of a specialised occupation. At the same time the industrial community must comprise a full complement of such specialised occupations, and must also be bound together in a system of communication sufficiently close and facile to allow the technological contents of all these occupations to be readily assimilated into a systematic whole. The industrial system so worked out need not be of the same extent as any one local group of the people who get their living by its use; but it seems to be required that if several local groups are effectively to be comprised in a single industrial system conditions of peace must prevail among them. Community of language seems also to be nearly necessary to the maintenance of such a system. Where the various local groups are on hostile terms, each will tend to have an industrial system of its own, with a technological character somewhat distinct from its neighbours.[62] If the degree of isolation is pronounced, so that traffic and communication do not run freely between groups, the size of the local group will limit the state of the industrial arts somewhat rigidly; and on the other hand a marked advance in the industrial arts, such as the domestication of crop plants or animals or the introduction of metals, is likely to bring about such a redistribution of population and industry as to increase the effective size of the community.[63]