Among the peoples on the lower levels of culture there prevails commonly a considerable degree of isolation, or even of estrangement. In a great degree each community is thrown on its own resources, and under these circumstances the size of the community may become a matter of decisive importance for the industrial arts. Where a serious decline in the numbers of any of these savage or barbarous peoples is recorded it is also commonly noted that they have suffered a concomitant decay in their technological knowledge and workmanship.[64] In view of these considerations it is probably safe to say that under settled conditions any community is, commonly, no larger than is required to keep up and carry forward the state of the industrial arts as it runs. The known evidence appears to warrant the generalisation that the state of the industrial arts is limited by the size of the industrial community, and that whenever a given community is broken up or suffers a serious diminution of numbers its technological heritage will deteriorate and dwindle even though it may apparently have been meagre enough before. The considerations recited above are matters of commonplace observation and might fairly be taken for granted without argument. But so much of current and recent theoretical speculation proceeds on tacit assumptions at variance with these commonplaces that it seems pertinent to recall them, particularly since they will come in as premises in later passages of the inquiry. * * * * *