The Wealth of Nations by Part 1 Chapter 10 Page 49

When the trade or practice becomes thoroughly established and well known, the competition reduces them to the level of other trades.

Secondly, this equality in the whole of the advantages and disadvantages of the different employments of labour and stock, can take place only in the ordinary, or what may be called the natural state of those employments.

The demand for almost every different species of labour is sometimes greater and sometimes less than usual. In the one case the advantages of the employment rise above, in the other they fall below the common level. The demand for country labour is greater at hay-time and harvest than during the greater part of the year; and wages rise with the demand. In time of war, when forty or fifty thousand sailors are forced from the merchant service